Add support for the /EP argument to clang-cl

This maps the /EP argument to both -E and -P.

Patch by Ehsan Akhgari!

Differential Reviion: http://reviews.llvm.org/D4133

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@210935 91177308-0d34-0410-b5e6-96231b3b80d8