glsl: add support for complie-time constant expressions

glsl: add support for complie-time constant expressions

This patch replaces the old interger constant qualifiers with either
the new ast_layout_expression type if the qualifier requires merging
or ast_expression if the qualifier can't have mulitple declarations
or if all but the newest qualifier is simply ignored.

We also update the process_qualifier_constant() helper to be
similar to the one in the ast_layout_expression class, but in
this case it will be used to process the ast_expression qualifiers.

Global shader layout qualifier validation is moved out of the parser
in this change as we now need to evaluate any constant expression
before doing the validation.

V2: Fix minimum value check for vertices (Emil)

Reviewed-by: Emil Velikov <emil.velikov@collabora.com>