clang-cl: ignore /showIncludes when combined with /E (PR20336)

Both /showIncludes and /E write to stdout. Allowing both results
in interleaved output and an error when double-closing the file
descriptor, intended to catch issues like this.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@213589 91177308-0d34-0410-b5e6-96231b3b80d8