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.
Unlike with synchronous functions, an async 'throws' function and an
async non-throwing function are not ABI-compatible. This would lead to
an ABI mismatch and runtime crash when overriding an `async throws`
function with an `async` one. Deal with this ABI incompatibility by
producing an appropriate thunk.
Fixes SR-15766 / rdar://problem/88035174.
Repurpose mangling operator `Y` as an umbrella operator that covers new attributes on function types. Free up operators `J`, `j`, and `k`.
```
async ::= 'Ya' // 'async' annotation on function types
sendable ::= 'Yb' // @Sendable on function types
throws ::= 'K' // 'throws' annotation on function types
differentiable ::= 'Yjf' // @differentiable(_forward) on function type
differentiable ::= 'Yjr' // @differentiable(reverse) on function type
differentiable ::= 'Yjd' // @differentiable on function type
differentiable ::= 'Yjl' // @differentiable(_linear) on function type
```
Resolves rdar://76299796.
Resilient witness tables and resilient class vtables are built from
descriptors. Make sure we reference the AsyncFunctionPointer of a
method implementation, and not the implementation itself, if the
method is async.
Part of rdar://problem/73625623.
