Refactor ternary operator parsing so that validation is done in
ParseContext and Intermediate's role is simply to create the node added
to the tree.

Remove partially bugged checks for null nodes as a part of this - in
error cases the parser doesn't typically add null nodes to the tree, but
rather always has a fallback to add a dummy node if parsing fails as a
method of recovery. When parsing ternary operators it should be
guaranteed that none of the parameter nodes is null.

Includes a better explanation of why ternary operators are not always
folded when only the condition is constant, and a test to make sure this
doesn't regress.

BUG=angleproject:952
TEST=WebGL conformance tests, angle_unittests

Change-Id: Icbcb721b5ab36cf314a16e79f9814aef1f355fa0
Reviewed-on: https://chromium-review.googlesource.com/265643Reviewed-by: Olli Etuaho <oetuaho@nvidia.com>
Tested-by: Olli Etuaho <oetuaho@nvidia.com>