[PATCH] Allow _MSC_EXTENSIONS macro on non-Windows targets

[Richard Smith]

  The patch looks good.

  What should we do about the iso646 alternative tokens (`and`, `or`, `compl`, ...)? These are technically not keywords (and this implementation of `__has_keyword` will return 0 for them), but they are also not usable as identifiers, and it would be useful for code to detect them (because in MSVC compat mode we need to treat them weirdly).

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