Downgrade diagnostic for uses of noasync declarations in async contexts in macro arguments

[hamishknight]

#80583 broke source compatibility for uses of noasync declarations in closure macro arguments passed to macros that have async function parameters:

@available(*, noasync)
func foo() {}

@freestanding(expression)
macro M(_ fn: () async -> Void) = #externalMacro(module: "", type: "")

#M {
  foo() // Legal in Swift 6, became an error under Swift 6 mode
}

We ought to downgrade the diagnostic to a warning for macro arguments until we get a new language mode.