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/+/2946115Reviewed-by: Ian Elliott <ianelliott@google.com>
Reviewed-by: Jonah Ryan-Davis <jonahr@google.com>
Commit-Queue: Shahbaz Youssefi <syoussefi@chromium.org>