[PATCH] Allow _MSC_EXTENSIONS macro on non-Windows targets

[Yunzhong Gao]

  I do not know. Are these alternative tokens pretty much set or are we expecting to add more in the future? Are
  they recognized as keywords only under MSVC compat mode?
  I guess we could have another __has_operator_token() test, but I can be convinced to rename __has_keyword()
  to mean any (non-attribute) special tokens.
  Also, this proposed implementation is not testing those objective-c "at" keywords (only a keyword if after an "@"
  in objc mode). I am not sure if this is a problem.

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