[CodeGen] Store the return value of the target function call to the

thunk's return value slot directly when the return type is an aggregate
instead of doing so via a temporary

This fixes PR45997 (https://bugs.llvm.org/show_bug.cgi?id=45997), which
is caused by a bug that has existed since we started passing and
returning C++ structs with ObjC strong pointer members (see
https://reviews.llvm.org/D44908) or structs annotated with trivial_abi
directly.

rdar://problem/63740936

Differential Revision: https://reviews.llvm.org/D82513