glsl: Construct gl_in with a location of -1.

We use a location of -1 for variables which don't have their own
assigned locations--this includes ir_variables which represent named
interface blocks.  Technically the location assigned to gl_in doesn't
matter, since gl_in is only accessed via its members (which have their
own locations).  But it's nice to be consistent.
glsl: Fix block name of built-in gl_PerVertex interface block.

Previously, we erroneously used the name "gl_in" for both the block
name and the instance name.
glsl: Refactor code for creating gl_PerVertex interface block.

Currently, we create just a single gl_PerVertex interface block for
geometry shader inputs.  In later patches, we'll also need to create
an interface block for geometry and vertex shader outputs.  Moving the
code into its own class will make reuse easier.
glsl: Construct gl_PerVertex interfaces for GS and VS outputs.

Although these interfaces can't be accessed directly by GLSL (since
they don't have an instance name), they will be necessary in order to
allow redeclarations of gl_PerVertex.
glsl: Account for location field when comparing interface blocks.

In commit e2660770731b018411fbe1620cacddaf8dff5287 (glsl: Keep track
of location for interface block fields), I neglected to update
glsl_type::record_key_compare to account for the fact that interface
types now contain location information.  As a result, interface types
that differ only by their location information would not be properly
distinguished.

At the moment this is not a problem, because the only interface block
in which location information != -1 is gl_PerVertex, and gl_PerVertex
is always created in the same way.  However, in the patches that
follow, we'll be adding new ways to create gl_PerVertex (by
redeclaring it), so we'll need location information to be handled
properly.
glsl: Refactor code to check that identifier names are valid.

GLSL reserves identifiers beginning with "gl_" or containing "__", but
we haven't been consistent about enforcing this rule.  This patch
makes a new function to check whether identifier names are valid.  In
the process it closes a loophole where we would previously allow
function argument names to contain "__".
!UPSTREAM: glsl: Don't allow unnamed interface blocks to redeclare variables.

Fixes piglit test
spec/glsl-1.50/compiler/unnamed-interface-block-elem-conflicts-with-prev-global.vert.

TODO:
- Some limited amount of redeclaration is allowed
glsl: Don't allow invalid identifiers as interface block names.

Note: we need to make an exception for the gl_PerVertex interface
block, since this is allowed to be redeclared.  Future patches will
make redeclaration of gl_PerVertex work properly.

Fixes piglit test
spec/glsl-1.50/compiler/interface-block-name-uses-gl-prefix.vert.
glsl: Don't allow invalid identifier names in struct/interface fields.

Note: we need to make an exception for the gl_PerVertex interface
block, since built-in variables are allowed to be redeclared inside
it.  Future patches will make redeclaration of gl_PerVertex work
properly.

Fixes piglit test
spec/glsl-1.50/compiler/unnamed-interface-block-elem-uses-gl-prefix.vert.
glsl: Don't allow invalid identifiers as interface block instance names.

Note: we need to make an exception for the gl_PerVertex interface
block, since in geometry shaders it is allowed to be redeclared with
the instance name gl_in.  Future patches will make redeclaration of
gl_PerVertex work properly.

Fixes piglit test
spec/glsl-1.50/compiler/interface-block-instance-name-uses-gl-prefix.vert.
glsl: Don't allow invalid identifiers as struct names.

Fixes piglit test
spec/glsl-1.10/compiler/struct/struct-name-uses-gl-prefix.vert.
glsl: Generalize processing of variable redeclarations.

This patch modifies the get_variable_being_redeclared() function so
that it no longer relies on the ast_declaration for the variable being
redeclared.  In future patches, this will allow
get_variable_being_redeclared() to be used for processing
redeclarations of the built-in gl_PerVertex interface block.
glsl: Support redeclaration of VS and GS gl_PerVertex output.

Fixes piglit test
spec/glsl-1.50/execution/redeclare-pervertex-subset-vs.
glsl: Catch redeclaration of interface block instance names at compile time.

From section 4.1.9 (Arrays) of the GLSL 4.40 spec (as of revision 7):

    However, unless noted otherwise, blocks cannot be redeclared;
    an unsized array in a user-declared block cannot be sized
    through redeclaration.

The only place where the spec notes that interface blocks can be
redeclared is to allow for redeclaration of built-in interface blocks
such as gl_PerVertex.  Therefore, user-defined interface blocks can
never be redeclared.  This is a clarification of previous intent (see
Khronos bug 10659:
https://cvs.khronos.org/bugzilla/show_bug.cgi?id=10659).

We were already preventing interface block redeclaration using the
same block name at compile time, but we weren't preventing interface
block redeclaration using the same instance name (and different block
names) at compile time.  And we weren't preventing an instance name
from conflicting with a previously-declared ordinary variable.

In practice the problem would be caught at link time, but only because
of a coincidence: since ast_interface_block::hir() wasn't doing any
checking to see if the instance name already existed in the shader, it
was creating a second ir_variable in the shader having the same name
but a different type.  Coincidentally, when the linker checked for
intrastage consistency of global variable declarations, it treated the
two declarations from the same shader as a conflict, so it reported a
link error.

But it seems dangerous to rely on that linker behaviour to catch
illegal redeclarations that really ought to be detected at compile
time.
glsl: Support redeclaration of GS gl_PerVertex input.