[Concurrency] Tune overloading to to allow sync overloads in async contexts

[DougGregor]

The existing overloading rules strongly prefer async functions within
async contexts, and synchronous functions in synchronous contexts.
However, when there are other differences in the
signature, particularly parameters of function type that differ in
async vs. synchronous, the overloading rule would force the use of the
synchronous function even in cases where the synchronous function
would be better. An example:

func f(_: (Int) -> Int) { }
func f(_: (Int) async -> Int) async { }

func g(_ x: Int) -> Int { -x }

func h() async {
  f(g) // currently selects async f, want to select synchronous f
}

Effect the semantics change by splitting the "sync/async mismatch"
score in the constraint system into an "async in sync mismatch" score
that is mostly disqualifying (because the call will always fail) and a
less-important score for "sync used in an async context", which also
includes conversion from a synchronous function to an asynchronous
one. This way, only synchronous functions are still considered within
a synchronous context, but we get more natural overloading behavior
within an asynchronous context. The end result is intended to be
equivalent to what one would get with reasync:

func f(_: (Int) async -> Int) async { ... }

Addresses rdar://74289867.

[DougGregor]

@swift-ci please smoke test

[DougGregor]

@swift-ci please test source compatibility