clang-cl: fix passing optimization level to cl.exe in /fallback mode

We were previously mostly passing it through, but -O0 and -O3 are not valid
options to cl.exe.

We should translate -O0 to /Od and -O3 to /Ox. -O{1,2,s} get passed through.

git-svn-id: http://llvm.org/svn/llvm-project/cfe/trunk@191323 91177308-0d34-0410-b5e6-96231b3b80d8