A few warnings were fixed:
- 2 sets of virtual flip/blit functions were colliding, so
  I added pure virtual function overrides so that both
  definitions appear within FrameBufferWin
- Moved a few variables within ASSERT inside the ASSERT
  statement in order to remove unused variable warnings
- Removed stack option from glslang.l and removed comments
  handling code, which is actually already done by the
  preprocessor (tested in dEQP)
- Removed unused yyscanner variable from glslang.l
- Ifdefed debug only code in main.cpp

Removed all related muted warnings from BUILD.gn files.

Change-Id: Idf9e7eed00431cc747b689b5d1931fd0c1e8d506
Reviewed-on: https://swiftshader-review.googlesource.com/8010Tested-by: Alexis Hétu <sugoi@google.com>
Reviewed-by: Nico Weber <thakis@google.com>
Reviewed-by: Nicolas Capens <capn@google.com>