Right now context does not know which specialization constant is used by
the shader. Whenever a specialization constant changes, we assume shader
program is using it, we always reach into vulkan driver to ask for a new
program. Instead we can track shader's usage of specialization constant
so that context can utilize this information to avoid recompile pipeline
program if an unused specialization constant has changed.

This CL implements the plumbing the usage bits form compiler to program
object, it does not actually utilize the usage bits to avoid unnecessary
compilation yet.

Bug: b/173461931
Change-Id: Iebc9d0638c17b1a282c8b6093ce6bae154246e57
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/2542866Reviewed-by: Ian Elliott <ianelliott@google.com>
Reviewed-by: Jamie Madill <jmadill@chromium.org>
Commit-Queue: Charlie Lao <cclao@google.com>