[clang] Make sure the same UsingType is searched and inserted (#79182)

When creating a new UsingType, the underlying type may change if it is a
declaration. This creates an inconsistency between the type searched and
type created. Update member and non-member Profile functions so that
they return the same ID.

Author: apolloww

clang/include/clang/AST/Type.h

@@ -4729,13 +4729,12 @@ class UsingType final : public Type,
   bool typeMatchesDecl() const { return !UsingBits.hasTypeDifferentFromDecl; }
 
   void Profile(llvm::FoldingSetNodeID &ID) {
-    Profile(ID, Found, typeMatchesDecl() ? QualType() : getUnderlyingType());
+    Profile(ID, Found, getUnderlyingType());
   }
   static void Profile(llvm::FoldingSetNodeID &ID, const UsingShadowDecl *Found,
                       QualType Underlying) {
     ID.AddPointer(Found);
-    if (!Underlying.isNull())
-      Underlying.Profile(ID);
+    Underlying.Profile(ID);
   }
   static bool classof(const Type *T) { return T->getTypeClass() == Using; }
 };

clang/test/AST/ast-dump-using.cpp

@@ -12,7 +12,13 @@ using a::S;
 typedef S f; // to dump the introduced type
 // CHECK:      TypedefDecl
 // CHECK-NEXT: `-ElaboratedType {{.*}} 'S' sugar
-// CHECK-NEXT:   `-UsingType {{.*}} 'a::S' sugar
-// CHECK-NEXT:     |-UsingShadow {{.*}} 'S'
+// CHECK-NEXT:   `-UsingType [[TYPE_ADDR:.*]] 'a::S' sugar
+// CHECK-NEXT:     |-UsingShadow [[SHADOW_ADDR:.*]] 'S'
+// CHECK-NEXT:     `-RecordType {{.*}} 'a::S'
+typedef S e; // check the same UsingType is reused.
+// CHECK:      TypedefDecl
+// CHECK-NEXT: `-ElaboratedType {{.*}} 'S' sugar
+// CHECK-NEXT:   `-UsingType [[TYPE_ADDR]] 'a::S' sugar
+// CHECK-NEXT:     |-UsingShadow [[SHADOW_ADDR]] 'S'
 // CHECK-NEXT:     `-RecordType {{.*}} 'a::S'
 }