With the SH_INIT_OUTPUT_VARIABLES option enabled, vertex and fragment
shader outputs get initialized with zeros at the beginning of main.
However, previous to this patch the lvalues in the binary expression did
not receive the correct type. This can lead to incorrect modifications
of the AST in subsequent stages or incorrect output code from the
translator. The patch addresses the issue by copying the type
information from the symbol table.

BUG=angleproject:2081
TEST=angle_unittests
TEST=angle_end2end_tests

Change-Id: I9e062376bcfad7d57b637a5248caebce1c9a0688
Reviewed-on: https://chromium-review.googlesource.com/544982
Commit-Queue: Olli Etuaho <oetuaho@nvidia.com>
Reviewed-by: Jamie Madill <jmadill@chromium.org>