glsl: Use a single shared namespace in the symbol table.

glsl: Use a single shared namespace in the symbol table.

As of 1.20, variable names, function names, and structure type names all
share a single namespace, and should conflict with one another in the
same scope, or hide each other in nested scopes.

However, in 1.10, variables and functions can share the same name in the
same scope.  Structure types, however, conflict with/hide both.

Fixes piglit tests redeclaration-06.vert, redeclaration-11.vert,
redeclaration-19.vert, and struct-05.vert.