[PATCH] Allow _MSC_EXTENSIONS macro on non-Windows targets

[David Majnemer]

  This change looks good to me, I'd like @rsmith's opinion on the spelling of `msvc_keywords ` before you commit it.

  If/when you do commit this, please make sure to commit this with a different message than the current one.

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