[clang-cl] Handle -O correctly

We had multiple bugs here:
- We didn't support multiple optimization options in one argument.
  e.g. -O2y-
- We didn't correctly expand -O[12dx] to their respective options.
- We treated -O1 as clang -O1 instead of clang -Os.
- We treated -Ox as clang -O3 instead of clang -O2.  In fact, cl's -Ox
  option is *less* powerful than cl's -O2 option despite -Ox described
  as "Full Optimization".

This fixes PR24003.

llvm-svn: 243261