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

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@194403 91177308-0d34-0410-b5e6-96231b3b80d8