[coro] Async coroutines: Allow more than 3 arguments in the dispatch function

We need to be able to call function pointers. Inline the dispatch
function.

Also inline the context projection function.

Transfer debug locations from the suspend point to the inlined functions.

Use the function argument index instead of the function argument in
coro.id.async. This solves any spurious use issues.

Coerce the arguments of the tail call function at a suspend point. The LLVM
optimizer seems to drop casts leading to a vararg intrinsic.

rdar://70097093

Differential Revision: https://reviews.llvm.org/D91098