clang-cl: Better error message when trying to compile stdin (PR18640)

We should suggest using /Tc or /Tp to set the input type, instead of
erroneously suggesting -x, which isn't a clang-cl flag.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@200362 91177308-0d34-0410-b5e6-96231b3b80d8