Clarify driver/frontend -fms-compatibility help text

Better describe the flag that enables drop-in MSVC compatibility, including
ability to parse MS standard headers. This is intended to distinguish it from
-fms-extensions, the more established and 'gentler' flag also supported by GCC.

The new wording matches up with the internal description introduced in r198936.
Still room for improvement (e.g. C++ is part of the product name, yet the flag
also applies to C) but it's a step forward from "Microsoft mode".

llvm-svn: 199247