glsl: Don't allow unnamed interface blocks to redeclare variables.

Note: some limited amount of redeclaration is actually allowed,
provided the shader is redeclaring the built-in gl_PerVertex interface
block.  Support for this will be added in future patches.

Fixes piglit test
spec/glsl-1.50/compiler/unnamed-interface-block-elem-conflicts-with-prev-global.vert.
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: Add an ir_variable::reinit_interface_type() function.

This will be used by future patches to change an ir_variable's
interface type when the gl_PerVertex built-in interface block is
redeclared.
glsl: Make it possible to remove a variable from the symbol table.

In later patches, we'll use this in order to implement the required
behaviour that after the gl_PerVertex interface block has been
redeclared, only members of the redeclared interface block may be
used.
glsl: Error check redeclarations of gl_PerVertex.

This patch verifies that:

- The gl_PerVertex input interface block may only be redeclared in a
  geometry shader, and that it may only be redeclared as gl_in[].

- The gl_PerVertex output interface block may only be redeclared in a
  vertex or geometry shader, and that it may only be redeclared as a
  non-array without an interface name.

- gl_PerVertex may not be redeclared as any other type of interface
  block (i.e. as a uniform interface block).

As a side-effect, the code now keeps track of what the previous
declaration of gl_PerVertex was--this will be needed in future
patches.

squash! glsl: Error check redeclarations of gl_PerVertex.
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.
glsl: Don't allow gl_PerVertex to be redeclared after it's been used.