Add __has_feature(attribute_availability_with_message).

This tests for the ability to include a "message" field in availability
attributes, like so:

  extern void ATSFontGetName(const char *oName)
    __attribute__((availability(macosx,introduced=8.0,deprecated=9.0,
                                message="use CTFontCopyFullName")));

This was actually supported in Clang 3.1, but we got a request for a
__has_feature so that header files can use this more safely. It's
unfortunate that the 3.1 release doesn't include this, however.

<rdar://problem/11886458>

llvm-svn: 160699