[coroutine] add nomerge function attribute to `llvm.coro.save`

It is illegal to merge two `llvm.coro.save` calls unless their
`llvm.coro.suspend` users are also merged. Marks it "nomerge" for
the moment.

This reverts D129025.

Alternative to D129025, which affects other token type users like WinEH.

Reviewed By: ChuanqiXu

Differential Revision: https://reviews.llvm.org/D129530