[PATCH] Allow _MSC_EXTENSIONS macro on non-Windows targets

[David Majnemer]

  I don't think I understand the problem you are trying to solve.

  If you want to determine whether or not MSVC keywords are enabled, I don't see why it wouldn't be unreasonable to add a `msvc_keywords` argument for `__has_extension`.

http://llvm-reviews.chandlerc.com/D3034