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

[Sylvestre Ledru]

  I just committed to make sure it will be in the 3.4 branch.
  However, if there is a consensus to revert this change, I won't mind doing it :)
   (as you can imagine, this commit goal is to simplify the transition from gcc to clang)

http://llvm-reviews.chandlerc.com/D2152

COMMIT
  http://llvm-reviews.chandlerc.com/rL194817