Teach the backend to make references to swift_async_extendedFramePointerFlags weak if it emits it

When references to the symbol `swift_async_extendedFramePointerFlags`
are emitted they have to be weak.

References to the symbol `swift_async_extendedFramePointerFlags` get
emitted only by frame lowering code. Therefore, the backend needs to track
references to the symbol and mark them weak.

Differential Revision: https://reviews.llvm.org/D115672