IRGen: fix how metadata is loaded for fast class casting

Pass the correct type to `emitHeapMetadataRefForHeapObject()`.
Fixes a runtime crash in case a class protocol is cast to a final swift class, where the actual object is an ObjectiveC class instance.

rdar://99626888

Author: eeckstein

lib/IRGen/GenCast.cpp

@@ -1108,7 +1108,7 @@ llvm::Value *irgen::emitFastClassCastIfPossible(IRGenFunction &IGF,
 
   // Load the isa pointer.
   llvm::Value *objMetadata = emitHeapMetadataRefForHeapObject(IGF, instance,
-      targetFormalType, GenericSignature(), /*suppress cast*/ true);
+      sourceFormalType, GenericSignature(), /*suppress cast*/ true);
   llvm::Value *rhs = IGF.Builder.CreateBitCast(objMetadata, IGF.IGM.Int8PtrTy);
 
   // return isa_ptr == metadata_ptr ? instance : nullptr

lib/IRGen/GenHeap.cpp

@@ -1917,7 +1917,8 @@ llvm::Value *irgen::emitHeapMetadataRefForHeapObject(IRGenFunction &IGF,
                                                      GenericSignature sig,
                                                      bool suppressCast) {
   ClassDecl *theClass = objectType.getClassOrBoundGenericClass();
-  if (theClass && isKnownNotTaggedPointer(IGF.IGM, theClass)) {
+  if ((theClass && isKnownNotTaggedPointer(IGF.IGM, theClass)) ||
+      !IGF.IGM.ObjCInterop) {
     auto isaEncoding = getIsaEncodingForType(IGF.IGM, objectType, sig);
     return emitLoadOfHeapMetadataRef(IGF, object,
                                      isaEncoding,

test/Casting/fast_class_casts.swift

@@ -11,7 +11,7 @@
 
 // 2. check if the generated IR looks like expected:
 
-// RUN: %target-swift-frontend -module-name=Main -I%t %s -emit-ir -g -o - | %FileCheck %s
+// RUN: %target-swift-frontend -module-name=Main -I%t %s -emit-ir -g -o - | %FileCheck %s -check-prefix=CHECK -check-prefix=CHECK-%target-objc-interop
 
 // REQUIRES: executable_test
 
@@ -48,6 +48,7 @@ func castToFinal(_ b: Classes.Base) -> Classes.Final? {
 }
 
 // CHECK-LABEL: define {{.*}} @"$s4Main24unconditionalCastToFinaly7Classes0E0CAC4BaseCF"
+// CHECK-NOT:     call {{.*}}@object_getClass
 // CHECK-NOT:     @swift_dynamicCastClass
 // CHECK:       }
 @inline(never)
@@ -64,6 +65,8 @@ func castToResilientFinal(_ b: ResilientClasses.Base) -> ResilientClasses.Final?
 }
 
 // CHECK-LABEL: define {{.*}} @"$s4Main19castProtocolToFinaly7Classes0E0CSgAC1P_pF"
+// CHECK-objc:    call {{.*}}@object_getClass
+// CHECK-nonobjc: load
 // CHECK-NOT:     @swift_dynamicCastClass
 // CHECK:       }
 @inline(never)