[Coroutines] Mark 'coroutine_handle<>::address' as always-inline

Close https://github.com/llvm/llvm-project/issues/65054

The direct issue is still the call to coroutine_handle<>::address()
after await_suspend(). Without optimizations, the current logic will put
the temporary result of await_suspend() to the coroutine frame since the
middle end feel the temporary is escaped from
coroutine_handle<>::address. To fix this fundamentally, we should wrap
the whole logic about await-suspend into a standalone function. See
https://github.com/llvm/llvm-project/issues/64945

And as a short-term workaround, we probably can mark
coroutine_handle<>::address() as always-inline so that the temporary
result may not be thought to be escaped then it won't be put on the
coroutine frame. Although it looks dirty, it is probably do-able since
the compiler are allowed to do special tricks to standard library
components.