Don't warn about the 'extern' in 'extern "C"' on a tag decl.   This is
usually useless, but not always.

llvm-svn: 128326
diff --git a/clang/lib/Sema/SemaDecl.cpp b/clang/lib/Sema/SemaDecl.cpp
index 5a4df40..59b4f5f 100644
--- a/clang/lib/Sema/SemaDecl.cpp
+++ b/clang/lib/Sema/SemaDecl.cpp
@@ -1870,9 +1870,14 @@
   if (emittedWarning || (TagD && TagD->isInvalidDecl()))
     return TagD;
 
+  // Note that a linkage-specification sets a storage class, but
+  // 'extern "C" struct foo;' is actually valid and not theoretically
+  // useless.
   if (DeclSpec::SCS scs = DS.getStorageClassSpec())
-    Diag(DS.getStorageClassSpecLoc(), diag::warn_standalone_specifier)
-      << DeclSpec::getSpecifierName(scs);
+    if (!DS.isExternInLinkageSpec())
+      Diag(DS.getStorageClassSpecLoc(), diag::warn_standalone_specifier)
+        << DeclSpec::getSpecifierName(scs);
+
   if (DS.isThreadSpecified())
     Diag(DS.getThreadSpecLoc(), diag::warn_standalone_specifier) << "__thread";
   if (DS.getTypeQualifiers()) {