The Invariant decoration is applied to variable declarations and block
members as necessary.  When invariant is applied to a gl_PerVertex
built-in, it's applied to the appropriate member in
DeclarePerVertexBlocks.

Bug: angleproject:4889
Change-Id: Idaa8d4ab92f975381a0237d372f0a4044ff983e0
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/2946116
Commit-Queue: Shahbaz Youssefi <syoussefi@chromium.org>
Reviewed-by: Jamie Madill <jmadill@chromium.org>
Reviewed-by: Tim Van Patten <timvp@google.com>