[cxx-interop] Fix crash in ASTMangler triggered by UsingShadowDecls

We did not handle this declaration kind. This PR introduces makes sure
we mangle it the same way we do for the target declaration.

Fixes swiftlang#82108.

Author: Gabor Horvath

lib/AST/ASTMangler.cpp

@@ -49,6 +49,7 @@
 #include "clang/AST/ASTContext.h"
 #include "clang/AST/Attr.h"
 #include "clang/AST/Decl.h"
+#include "clang/AST/DeclCXX.h"
 #include "clang/AST/DeclObjC.h"
 #include "clang/AST/DeclTemplate.h"
 #include "clang/AST/Mangle.h"
@@ -3080,7 +3081,7 @@ void ASTMangler::appendAnyGenericType(const GenericTypeDecl *decl,
 
   auto *nominal = dyn_cast<NominalTypeDecl>(decl);
 
-  if (nominal && isa<BuiltinTupleDecl>(nominal))
+  if (isa_and_nonnull<BuiltinTupleDecl>(nominal))
     return appendOperator("BT");
 
   // Check for certain standard types.
@@ -3132,6 +3133,12 @@ void ASTMangler::appendAnyGenericType(const GenericTypeDecl *decl,
       return false;
     }
 
+    // Mangle `Foo` from `namespace Bar { class Foo; } using Bar::Foo;` the same
+    // way as if we spelled `Bar.Foo` explicitly.
+    if (const auto *usingShadowDecl =
+            dyn_cast<clang::UsingShadowDecl>(namedDecl))
+      namedDecl = usingShadowDecl->getTargetDecl();
+
     // Mangle ObjC classes using their runtime names.
     auto interface = dyn_cast<clang::ObjCInterfaceDecl>(namedDecl);
     auto protocol = dyn_cast<clang::ObjCProtocolDecl>(namedDecl);

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

@@ -69,3 +69,8 @@ module MembersTransitive {
   header "members-transitive.h"
   requires cplusplus
 }
+
+module UsingFromNamespace {
+  header "using-from-namespace.h"
+  requires cplusplus
+}

test/Interop/Cxx/namespace/Inputs/using-from-namespace.h

@@ -0,0 +1,13 @@
+#pragma once
+
+namespace Test {
+  class Foo {};
+  namespace Test2 {
+  class Bar {};
+  } // namespace Test2
+
+  using Test2::Bar;
+} // namespace Test
+
+using Test::Bar;
+using Test::Foo;

test/Interop/Cxx/namespace/using-from-namespace.swift

@@ -0,0 +1,6 @@
+// RUN: %target-swift-emit-irgen -I %S/Inputs/ -cxx-interoperability-mode=default -g %s
+
+import UsingFromNamespace
+
+func f(_ foo: Foo) {}
+func g(_ bar: Bar) {}