A new qualifier (EvqSpecConst) is added so that specialization constants
can be declared in tree.

This enables TVariable references to specialization constants to be
validated, which were in fact invalid as every reference declared a new
variable.  That is fixed.

Bug: angleproject:4889
Change-Id: I1711e41cbc5a1260843d2d004d3568bdae11e963
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/2941451Reviewed-by: Tim Van Patten <timvp@google.com>
Reviewed-by: Jamie Madill <jmadill@chromium.org>
Commit-Queue: Shahbaz Youssefi <syoussefi@chromium.org>