clang-cl: Support the /fp options (PR23112)

This hooks up the /fp options as aliases for -f[no-]fast-math and
-f[no]-trapping-math. It probably doesn't match cl.exe's behaviour
completely (e.g. LLVM is currently never as precise as /fp:precise),
but it's close enough.

Differential revision: http://reviews.llvm.org/D8909

llvm-svn: 234449