In case folding a ternary op or a comma op would change the qualifier
of the expression, the folding is deferred to a separate traversal
step.

After this there are no more cases where the type of a TIntermSymbol
node needs to differ from the type of the variable it is referring to.
There are still some cases where some parts of TIntermSymbol type are
changed while keeping the TVariable type the same though, like when
assigning array size to gl_PerVertex nodes or sanitizing qualifiers of
struct declarations.

BUG=angleproject:2267
TEST=angle_unittests, angle_end2end_tests

Change-Id: I1501c8d361f5f765f43ca810d1b7248d9e2c5986
Reviewed-on: https://chromium-review.googlesource.com/850672
Commit-Queue: Olli Etuaho <oetuaho@nvidia.com>
Reviewed-by: Corentin Wallez <cwallez@chromium.org>