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

Summary:
Currently with clang:
$ clang -O20 foo.c
error: invalid value '20' in '-O20'

With the patch:
$ clang -O20 foo.c
warning: optimization level '-O20' is unsupported; using '-O3' instead.
1 warning generated.

This matches the gcc behavior (with a warning added)

Pass all tests:
Testing: 0 .. 10.. 20.. 30.. 40.. 50.. 60.. 70.. 80.. 90..
Testing Time: 94.14s
  Expected Passes    : 6721
  Expected Failures  : 20
  Unsupported Tests  : 17

(which was not the case of http://llvm-reviews.chandlerc.com/D2125)

Differential Revision: http://llvm-reviews.chandlerc.com/D2212