[PATCH] Option parsing: support case-insensitive option matching.

[Rui Ueyama]

  I reverted this patch as it broke Windows build. strcasecmp and strncasecmp are not C/C++ standard functions, and Windows does not define them. I'll instead use tolower(), which is a C++ standard function, as I did in http://llvm-reviews.chandlerc.com/differential/diff/3825/ and re-submit.

http://llvm-reviews.chandlerc.com/D1485

COMMIT
  http://llvm-reviews.chandlerc.com/rL189416