[Coroutines] Handle the writes to promise alloca prior to

llvm.coro.begin

Previously we've taken care of the writes to allocas prior to llvm.coro.begin.
However, since the promise alloca is special so that we never handled it
before. For the long time, since the programmers can't access the
promise_type due to the c++ language specification, we still failed to
recognize the problem until a recent report:
https://github.com/llvm/llvm-project/issues/57861

And we've tested many codes that the problem gone away after we handle
the writes to the promise alloca prior to @llvm.coro.begin()
prope until a recent report:
https://github.com/llvm/llvm-project/issues/57861

And we've tested many codes that the problem gone away after we handle
the writes to the promise alloca prior to @llvm.coro.begin() properly.

Closes https://github.com/llvm/llvm-project/issues/57861