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.
WIP
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.