[ClangImporter] Suffix ambiguous protocol names if C++ interop is enabled

The Clang Importer when C++ interop is not enabled, disambigate an Obj-C
class and protocol that are named the same by appending `Protocol` to
the protocol. This was not happening when C++ interop was enabled, but
should also apply to Obj-C++ modules.

The fix is providing an starting scope for the search, which the C++
name lookup need to actually find the similarly named counterpart.

Includes a test to avoid this problem creeping in again, and locally it
did not break any other tests.

Author: drodriguez

lib/ClangImporter/ImportName.cpp

@@ -999,15 +999,20 @@ bool NameImporter::hasNamingConflict(const clang::NamedDecl *decl,
   lookupResult.setAllowHidden(true);
   lookupResult.suppressDiagnostics();
 
-  if (clangSema.LookupName(lookupResult, /*scope=*/nullptr)) {
+  if (clangSema.LookupName(lookupResult, /*scope=*/clangSema.TUScope)) {
     if (std::any_of(lookupResult.begin(), lookupResult.end(), conflicts))
       return true;
   }
 
-  lookupResult.clear(clang::Sema::LookupTagName);
-  if (clangSema.LookupName(lookupResult, /*scope=*/nullptr)) {
-    if (std::any_of(lookupResult.begin(), lookupResult.end(), conflicts))
-      return true;
+  // No need to lookup tags if we are using C++ mode.
+  if (!clang::LangStandard::getLangStandardForKind(
+          clangSema.getLangOpts().LangStd)
+          .isCPlusPlus()) {
+    lookupResult.clear(clang::Sema::LookupTagName);
+    if (clangSema.LookupName(lookupResult, /*scope=*/nullptr)) {
+      if (std::any_of(lookupResult.begin(), lookupResult.end(), conflicts))
+        return true;
+    }
   }
 
   return false;

test/Interop/Cxx/class/Inputs/class-protocol-name-clash.h

@@ -0,0 +1,5 @@
+@protocol TheClashingName
+@end
+
+@interface TheClashingName <TheClashingName>
+@end

test/Interop/Cxx/class/Inputs/module.modulemap

@@ -87,3 +87,8 @@ module InvalidNestedStruct {
   header "invalid-nested-struct.h"
   requires cplusplus
 }
+
+module ClassProtocolNameClash {
+  header "class-protocol-name-clash.h"
+  requires cplusplus
+}

test/Interop/Cxx/class/class-protocol-name-clash.swift

@@ -0,0 +1,7 @@
+// RUN: %target-swift-frontend %s -c -enable-cxx-interop -I %S/Inputs
+//
+// REQUIRES: objc_interop
+
+import ClassProtocolNameClash
+
+class Subclass : TheClashingName {}