dEQP tests enforce that the defined operator should be parsed even when
it is generated as a result of macro expansion, even though this is
undefined according to the C++ preprocessor spec.

Implement support for this by putting the parsing for the defined
operator inside MacroExpander. The operator gets processed right after
it is generated by macro expansion. Parsing the defined operator is
toggled with a boolean according to the context where MacroExpander
is used.

BUG=angleproject:989
TEST=angle_unittests,
     dEQP-GLES3.functional.shaders.preprocessor.* - 2 tests start passing:
     dEQP-GLES3.functional.shaders.preprocessor.conditional_inclusion.basic_2*

Change-Id: I780e63bd4558253657d898685d62339017564a06
Reviewed-on: https://chromium-review.googlesource.com/300970Reviewed-by: Jamie Madill <jmadill@chromium.org>
Tested-by: Olli Etuaho <oetuaho@nvidia.com>