Supported older compilers don't enable this feature by default, so it can't be
omitted from the `_Concurrency` module's flags (regression from
https://github.com/swiftlang/swift/pull/74543).
Additionally, remove `@_allowFeatureSuppression(IsolatedAny)` from all
declarations. We no longer need to support compilers that don't have the
`IsolatedAny` feature, so the suppression is superfluous and the alternative
branches didn't actually build anyways. _Additionally_, the suppressible
feature logic could not handle suppressing `IsolatedAny` simultaneously with
`SendingArgsAndResults`, resulting in a broken interface because `sending` was
used outside `#if $SendingArgsAndResults` guards.
While the new parameter is added in a compatible way where code which
does not refer to it will get a defaulted nil value; since we refer to a
new parameter name in source, we need to guard it with a language
feature -- as old compilers will not have this new name available.
This should prevent a potential condfail issue.
Concurrency runtime expects discarding task operation entrypoint
function not to have result type, but the current SILGen
implementation generates reabstraction thunk to convert `() -> Void`
to `() -> T` for the operation function.
Since the `T` is always `Void` for DiscardingTG, the mismatch of result
type expectation does not cause any problem on most platforms, but the
signature mismatch causes a problem on WebAssembly.
This patch introduces new builtin operations for creating discarding
task, which always takes `() -> Void` as the operation function type.
