This patch intends to fix a compile error in ANGLE GLSL compiler when
parsing an expression with inputs which has interpolation qualifiers
('flat', 'smooth' and 'centroid').

The compiler should report a compile error when a shader input with
interpolation qualifier is used as a l-value.

BUG=angleproject:2140
TEST=angle_unittests

Change-Id: I7c059d53bf001ac31d34519a98e5289797833ce7
Reviewed-on: https://chromium-review.googlesource.com/640075
Commit-Queue: Olli Etuaho <oetuaho@nvidia.com>
Reviewed-by: Jamie Madill <jmadill@chromium.org>