glsl: do not lookup struct types by typename

glsl: do not lookup struct types by typename

This changes the logic during the conversion of the declaration list

   struct S {
      ...
   } v;

from AST to IR, but should not change the end result.

When assigning the type of v, instead of looking `S' up in the symbol
table, we read the type from the member variable of ast_struct_specifier.

This change is necessary for the subsequent change to how anonymous types
are handled.

v2: remove a type override when redefining a structure; should be
    the same type in that case anyway

Reviewed-by: Timothy Arceri <tarceri@itsqueeze.com>