[PATCH] Using an invalid -O falls back on -O3 instead of an error

[Renato Golin]

  Oh, I just realised that -O4 is -O3 + LTO. Will this function run on -O4, or will that be already be reverted back to -O3+LTO at the time when this function runs?

http://llvm-reviews.chandlerc.com/D2125

BRANCH
  /cfe/trunk

ARCANIST PROJECT
  clang