Add constructors for TMemoryQualifier and TLayoutQualifier. These replace the ::create methods, to facilitate constexpr versions. Also adds a constructor of WorkGroupSize that can be made to be constexpr. Bug: angleproject:1432 Change-Id: I413d1a56102c94d6a383775110a2b850a7532ea0 Reviewed-on: https://chromium-review.googlesource.com/776279 Commit-Queue: Jamie Madill <jmadill@chromium.org> Reviewed-by: Kai Ninomiya <kainino@chromium.org>