Translator: Allow invariant on I/O block members The GLSL ES 3.2 spec says: > Only variables output from a shader can be candidates for invariance. And: > All uses of invariant must be at global scope or on block members, and > before any use of the variables being declared as invariant. As a result, it's possible to specify `invariant` on a member of an output I/O block. Bug: angleproject:4889 Change-Id: I8822ed214813db8424d6a67dfef7f069a448b19d Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/2946115 Reviewed-by: Ian Elliott <ianelliott@google.com> Reviewed-by: Jonah Ryan-Davis <jonahr@google.com> Commit-Queue: Shahbaz Youssefi <syoussefi@chromium.org>