Use the `%target-swift-5.1-abi-triple` substitution to compile the tests for
deployment to the minimum OS versions required for use of _Concurrency APIs,
instead of disabling availability checking.
If LLVM optimizations are to be disabled, we cannot just not run all LLVM passes, because there are some mandatory LLVM passes, like coro splitting.
Instead, just run the -O0 LLVM pipeline if -disable-llvm-optzns is used.
Fixes compiler crashes if -disable-llvm-optzns is used.
Note: if one wants to see the output of IRGen, -emit-irgen can be used.
The `coro.end.async` intrinsic allow specifying a function that is to be
tail-called as the last thing before returning.
LLVM lowering will inline the `must-tail-call` function argument to
`coro.end.async`. This `must-tail-call` function can contain a
`musttail` call.
```
define @my_must_tail_call_func(void (*)(i64) %fnptr, i64 %args) {
musttail call void %fnptr(i64 %args)
ret void
}
define @async_func() {
...
coro.end.async(..., @my_must_tail_call_func, %return_continuation, i64 %args)
unreachable
}
```
