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: 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.