[Coroutines] Look for dbg.declare for temp spills

A temp spill may not have direct dbg.declare attached. This can cause problem for debugger when it wants to print the value in resume/destroy/cleanup functions.

In particular, we found this happening to "this" pointer that a temp is used to store its value in entry block and spilled later.

Reviewed By: ChuanqiXu

Differential Revision: https://reviews.llvm.org/D146543