glsl: Add single declaration variables to the symbol table too.

glsl: Add single declaration variables to the symbol table too.

The lexer/parser use a symbol table to classify identifiers as
variables, functions, or structure types.

For some reason, we neglected to add variables in simple declarations
such as

    int x = 5;

but did add subsequent variables in multi-declarations:

    int x = 5, y = 6; // y gets added, but not x, for some reason

Fixes four dEQP-GLES2.functional.shaders.scoping.valid subcases:
- local_int_variable_hides_struct_type_vertex
- local_int_variable_hides_struct_type_fragment
- local_struct_variable_hides_struct_type_vertex
- local_struct_variable_hides_struct_type_fragment

Signed-off-by: Kenneth Graunke <kenneth@whitecape.org>
Reviewed-by: Timothy Arceri <timothy.arceri@collabora.com>