[PATCH] Allow _MSC_EXTENSIONS macro on non-Windows targets

[Yunzhong Gao]

  Hi David,
  Thanks for reviewing the change. I might need some clarifications.
  I tried the sample codes from the stackoverflow link (thanks for doing the
  research!), and it appears that we do not implement this bad behavior from MSVC
  even with "clang -cc1 -triple i686-pc-win32 -fms-extensions". So I deduce that
  "clang -fms-extensions" will behave differently than "cl /Ze" on Windows.
  On the other hand, this proposed patch is only making -fms-extensions behave
  the same on Windows and non-Windows targets. Since _MSC_EXTENSIONS is currently
  defined on Windows target, I guess programmers will already be surprised if
  they are using this macro to test MSVC-specific behaviors?
  - Gao.

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