This unifies the behavior across the compiler and rest of ANGLE - for
example, one can use #define ANGLE_TEST_CONFIG to disable UNIMPLEMENTED
asserts in both the compiler and the rest of ANGLE. Compiler traces from
asserts also go to the same TRACE_OUTPUT_FILE as other traces instead of
being directed through ParseContext.

The compiler build already includes the common sources, so no changes to
build config are needed.

The original version of this change was reverted due to release mode
build issues. This version adds UNUSED_ASSERTION_VARIABLE where needed.

TEST=angle_unittests, angle_end2end_tests,
     dEQP-GLES3.functional.shaders.*
BUG=angleproject:983

Change-Id: I36929020a04251b8bc834fbb3c069e10128c3082
Reviewed-on: https://chromium-review.googlesource.com/267411Reviewed-by: Jamie Madill <jmadill@chromium.org>
Reviewed-by: Olli Etuaho <oetuaho@nvidia.com>
Tested-by: Olli Etuaho <oetuaho@nvidia.com>