We need to generate a compiler error only when the vertex index
of an output variable as an l-value, in a tessellation control shader,
is not the indentifier gl_InvocationID.

Bug: angleproject:5557
Tests: dEQP-GLES31.functional.tessellation.shader_input_output.barrier
Change-Id: Ica075b8d85916caa228f057b203ae81b27f65e2d
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/2758915Reviewed-by: Jamie Madill <jmadill@chromium.org>
Reviewed-by: Shahbaz Youssefi <syoussefi@chromium.org>
Commit-Queue: Mohan Maiya <m.maiya@samsung.com>