glsl: add new type for compile time constants

glsl: add new type for compile time constants

In this patch we introduce a new ast type for holding the new
compile-time constant expressions. The main reason for this is that
we can no longer do merging of layout qualifiers before they have been
converted into GLSL IR so we need to store them to be proccessed later.

The new type has two helper functions:

- process_qualifier_constant()

 Used to merge and then evaluate qualifier expressions

- merge_qualifier()

 Simply appends a qualifier to a list to be merged later by
 process_qualifier_constant()

In order to avoid cascading error messages the process_qualifier_constant()
helpers return a bool

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