glsl: Propagate explicit binding information from the AST all the way to the linker

glsl: Propagate explicit binding information from the AST all the way to the linker

Information about the binding was not being properly communicated from
the front-end compiler to the linker.  As a result, the linker never
knew that any UBOs had explicit bindings!

Fixes the piglit test arb_shading_language_420pack-binding-layout.

Signed-off-by: Ian Romanick <ian.d.romanick@intel.com>
Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=76323
Reviewed-by: Kenneth Graunke <kenneth@whitecape.org>
Tested-by: github@socker.lepus.uberspace.de [v0]
Cc: "10.1" <mesa-stable@lists.freedesktop.org>
Cc: github@socker.lepus.uberspace.de