Introduce some infrastructure for adding C attributes with [[]] syntax.

This patch adds support to the attribute tablegen for specifying a [[]] attribute is allowed in C mode. This patch also adds the annotate attribute to the list of double square bracket attributes we support in C mode.

Eventually, I anticipate that this logic will be reversed (you have to opt out of allowing an attribute in C rather than opting in), but I want to see how the design plays out as more attributes are considered.

llvm-svn: 321763
diff --git a/clang/utils/TableGen/ClangAttrEmitter.cpp b/clang/utils/TableGen/ClangAttrEmitter.cpp
index b0e2ddd..2a8b891 100644
--- a/clang/utils/TableGen/ClangAttrEmitter.cpp
+++ b/clang/utils/TableGen/ClangAttrEmitter.cpp
@@ -87,6 +87,8 @@
     } else if (Variety == "Clang") {
       Ret.emplace_back("GNU", Name, "", false);
       Ret.emplace_back("CXX11", Name, "clang", false);
+      if (Spelling->getValueAsBit("AllowInC"))
+        Ret.emplace_back("C2x", Name, "clang", false);
     } else
       Ret.push_back(FlattenedSpelling(*Spelling));
   }