The "#pragma STDGL invariant(all)" directive should only be applied to
shader output vars. This change also removes the workaround
SH_DONT_REMOVE_INVARIANT_FOR_FRAGMENT_INPUT which is no longer needed.

This change fixes two tests that were incorrectly assuming that the
pragma would be applied to inputs: GLSLTest.InvariantAll[Both|In].

Bug: angleproject:1293
Bug: angleproject:3285
Change-Id: I4eb03fa89fbc7c560150ee0cc32382024b0cb3e3
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/1558678Reviewed-by: Jamie Madill <jmadill@chromium.org>
Reviewed-by: Tobin Ehlis <tobine@google.com>
Commit-Queue: Tobin Ehlis <tobine@google.com>