Add a command-line flag to control the Swift extended async frame info.

Introduce a new command-line flag `-swift-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`: the default, always set the bit statically, regardless of deployment
target.
* `never`: never set the bit, regardless of deployment target.

Patch by Doug Gregor <dgregor@apple.com>

Reviewed By: doug.gregor

Differential Revision: https://reviews.llvm.org/D109392