IRGen: Fix conversion of qualified Clang TypeDecls

... or something. This manifested as a crash when emitting an @objc thunk
for an NS_STRING_ENUM type.

Fixes <rdar://problem/35140812>.

Author: slavapestov

lib/IRGen/GenClangType.cpp

@@ -705,7 +705,8 @@ clang::CanQualType ClangTypeConverter::convert(IRGenModule &IGM, CanType type) {
     if (auto clangDecl = decl->getClangDecl()) {
       if (auto clangTypeDecl = dyn_cast<clang::TypeDecl>(clangDecl)) {
         auto &ctx = IGM.getClangASTContext();
-        return ctx.getCanonicalType(ctx.getTypeDeclType(clangTypeDecl));
+        return ctx.getCanonicalType(ctx.getTypeDeclType(clangTypeDecl))
+            .getUnqualifiedType();
       } else if (auto ifaceDecl = dyn_cast<clang::ObjCInterfaceDecl>(clangDecl)) {
         auto &ctx = IGM.getClangASTContext();
         auto clangType  = ctx.getObjCInterfaceType(ifaceDecl);

test/IRGen/Inputs/clang_string_enum.h

@@ -0,0 +1,5 @@
+#import <Foundation/Foundation.h>
+
+typedef NSString * const PandaStyle NS_STRING_ENUM;
+
+extern PandaStyle PandaStyleCute;

test/IRGen/clang_string_enum.swift

@@ -0,0 +1,12 @@
+// RUN: %empty-directory(%t)
+// RUN: %target-swift-frontend -emit-ir %s -import-objc-header %S/Inputs/clang_string_enum.h > /dev/null
+
+// REQUIRES: objc_interop
+
+import Foundation
+
+class PandaCub : NSObject {}
+
+extension PandaCub {
+  @objc func cuddle(_: PandaStyle) { }
+}