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: invalid value '20' in '-O20'. Fall back on value '3'

Reviewers: rengolin, hfinkel

Reviewed By: rengolin

CC: cfe-commits, hfinkel, rengolin

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