Rename nonInitErrorCheck to declareVariable to clarify that it declares
variables. Merge arrayErrorCheck with that so that logic that is common
between array and non-array declarations is only in one place. This
will simplify adding array initializer handling. This also enables
redeclaring gl_LastFragData using ESSL3 array type syntax.

Comments in executeInitializer claimed that the TVariable object was
needed for error recovery, but that was not actually true, so it can also
use the new declareVariable method. Make "variable" a local variable
instead of a parameter to executeInitializer, since the parameter was
never used by callers of the function.

TEST=angle_unittests, WebGL conformance tests
BUG=angleproject:941

Change-Id: Ie133be62afc3e1f997370803cf21cada4e738935
Reviewed-on: https://chromium-review.googlesource.com/264674Tested-by: Olli Etuaho <oetuaho@nvidia.com>
Reviewed-by: Jamie Madill <jmadill@chromium.org>