Check that layout qualifiers may only appear in declarations In the shading language grammar, layout qualifiers may appear in loop conditions and function declarations, but this is not valid ESSL 3.00 according to section 4.3.8. TEST=angle_unittests, dEQP-GLES3.functional.shaders.*layout* BUG=angleproject:1064 Change-Id: I73f805247bf6678b953f2e1d45a23391e9eacb96 Reviewed-on: https://chromium-review.googlesource.com/285040 Tested-by: Olli Etuaho <oetuaho@nvidia.com> Reviewed-by: Jamie Madill <jmadill@chromium.org> Reviewed-by: Zhenyao Mo <zmo@chromium.org>