Uniform block members may be structs containing matrices, and there's
no explicit mention in the spec that the matrix qualifiers would only
be valid for matrix fields. Only generate a warning when an extraneous
matrix layout qualifier is used.

TEST=dEQP-GLES3.functional.shaders.*layout* (all pass with this patch)
BUG=angleproject:1058

Change-Id: Ifb2f2392d84100d997cc2d68fd55e9ad17a17cea
Reviewed-on: https://chromium-review.googlesource.com/284112Reviewed-by: Zhenyao Mo <zmo@chromium.org>
Tested-by: Olli Etuaho <oetuaho@nvidia.com>