glsl: Make it possible to disable a variable in the symbol table.

glsl: Make it possible to disable a variable in 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.

v2: Update the function name and comment to clarify that we aren't
actually removing the variable from the symbol table, just disabling
it.

Reviewed-by: Ian Romanick <ian.d.romanick@intel.com>