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

clang/lib/Lex/PPMacroExpansion.cpp

@@ -619,6 +619,7 @@ static bool HasFeature(const Preprocessor &PP, const IdentifierInfo *II) {
            .Case("address_sanitizer", LangOpts.AddressSanitizer)
            .Case("attribute_analyzer_noreturn", true)
            .Case("attribute_availability", true)
+           .Case("attribute_availability_with_message", true)
            .Case("attribute_cf_returns_not_retained", true)
            .Case("attribute_cf_returns_retained", true)
            .Case("attribute_deprecated_with_message", true)

clang/test/Sema/attr-availability.c

@@ -37,3 +37,9 @@ void f6(int) __attribute__((availability(ios,deprecated=4.0))); // expected-warn
 void f7(int) __attribute__((availability(ios,introduced=2.0)));
 void f7(int) __attribute__((availability(ios,deprecated=3.0))); // expected-note {{previous attribute is here}}
 void f7(int) __attribute__((availability(ios,deprecated=4.0))); // expected-warning {{availability does not match previous declaration}}
+
+
+// <rdar://problem/11886458>
+#if !__has_feature(attribute_availability_with_message)
+# error "Missing __has_feature"
+#endif