[clang-cl] Print /showIncludes to stderr, if used in combination with /E, /EP or /P

[clang-cl] Print /showIncludes to stderr, if used in combination with /E, /EP or /P

This replicates 'cl.exe' behavior and allows for both preprocessor output and
dependency information to be extraced with a single compiler invocation.

This is especially useful for compiler caching with tools like Mozilla's sccache.

See: https://github.com/mozilla/sccache/issues/246

Patch By: fxb

Differential Revision: https://reviews.llvm.org/D46394

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@331533 91177308-0d34-0410-b5e6-96231b3b80d8