[MS ABI] Ignore transparent contexts when determining the effective context

We didn't skip over extern "C++" contexts, causing us to mangle things
which don't need to be mangled.

llvm-svn: 270089

clang/lib/AST/MicrosoftMangle.cpp

@@ -94,7 +94,7 @@ static const DeclContext *getEffectiveDeclContext(const Decl *D) {
     return getEffectiveDeclContext(cast<Decl>(DC));
   }
 
-  return DC;
+  return DC->getRedeclContext();
 }
 
 static const DeclContext *getEffectiveParentContext(const DeclContext *DC) {

clang/test/CodeGenCXX/mangle-ms.cpp

@@ -4,6 +4,11 @@
 int a;
 // CHECK-DAG: @"\01?a@@3HA"
 
+extern "C++" {
+static int __attribute__((used)) ignore_transparent_context;
+// CHECK-DAG: @ignore_transparent_context
+}
+
 namespace N {
   int b;
 // CHECK-DAG: @"\01?b@N@@3HA"