[Concurrency] Add an API in the Concurrency library for extracting isolation from a dynamically isolated function value.

[hborla]

Extracting the isolation value from a function whose type is @isolated(any) needs a surface syntax. There's already a builtin, so for now I've exposed this with the following function in the Concurrency library, copied from the _Concurrency swiftinterface in my local build to demonstrate availability and compiler feature guards:

#if compiler(>=5.3) && $ParameterPacks && $IsolatedAny
@available(macOS 10.15, iOS 13.0, watchOS 6.0, tvOS 13.0, *)
public func extractIsolation<each Arg, Result>(_ fn: @escaping @isolated(any) (repeat each Arg) async throws -> Result) -> (any _Concurrency.Actor)?
#endif

Note that I needed to fix a few small issues in the mandatory SILOptimizer passes, as this change introduces the first use of Builtin.extractFunctionIsolation / FunctionExtractIsolationInst.

[hborla]

@swift-ci please smoke test

[ktoso]

Can we add a test with myActor being a distributed actor? Doug did the work for #isolated to convert, but would it work here as well?

[hborla]

I'll add a test for this separately; it'll work as long as the type checker erasure to an @isolated(any) function type works when the function has an isolated distributed actor parameter. The type checker support was added in #71433, this PR just exposes a surface syntax for Builtin.extractFunctionIsolation

[ktoso]

Sounds good, I'm just always nervous when we add such surface API that we might break it if not covered with DA tests. Thanks in advance, should be quick one to add :)

[hborla]

@swift-ci please smoke test