IRGen: Make type(of:) behavior consistent in ObjC bridged contexts.

When we use type(of: x) on a class in an ObjC bridged context, the optimizer turns this into a SIL `value_metatype @objc` operation, which is supposed to get the dynamic type of the object as an ObjC class. This was previously lowered by IRGen into a `object_getClass` call, which extracts the isa pointer from the object, but is inconsistent with the `-class` method in ObjC or with the Swift-native behavior, which both look through artificial subclasses, proxies, and so on. This inconsistency led to observably different behavior between debug and release builds and between ObjC-bridged and native entry points, so provide an alternative runtime entry point that replicates the behavior of getting a native Swift class. Fixes SR-7258.

include/swift/Runtime/RuntimeFunctions.def

@@ -869,6 +869,12 @@ FUNCTION(GetObjCClassFromMetadata, swift_getObjCClassFromMetadata, C_CC,
          ARGS(TypeMetadataPtrTy),
          ATTRS(NoUnwind, ReadNone))
 
+// Metadata *swift_getObjCClassFromObject(id object);
+FUNCTION(GetObjCClassFromObject, swift_getObjCClassFromObject, C_CC,
+         RETURNS(ObjCClassPtrTy),
+         ARGS(ObjCPtrTy),
+         ATTRS(NoUnwind, ReadNone))
+
 // MetadataResponse swift_getTupleTypeMetadata(MetadataRequest request,
 //                                             TupleTypeFlags flags,
 //                                             Metadata * const *elts,