Add a new frontend flag `-fswift-async-fp={auto|always|never}`

Summary:
Introduce a new frontend flag `-fswift-async-fp={auto|always|never}`
that controls how code generation sets the Swift extended async frame
info bit. There are three possibilities:

* `auto`: which determines how to set the bit based on deployment target, either
statically or dynamically via `swift_async_extendedFramePointerFlags`.
* `always`: default, always set the bit statically, regardless of deployment
target.
* `never`: never set the bit, regardless of deployment target.

Differential Revision: https://reviews.llvm.org/D109451