[Concurrency] Don't attempt to hop to executor inside default argument generators and stored property initializers.

[hborla]

Otherwise, the compiler will fail an assertion or crash in IRGen because there's a suspension point in a synchronous function. For example:

@MainActor func onMain() -> Int { 0 }

@MainActor func useDefault(_ x: Int = onMain()) -> Int { x }

@MainActor func anotherDefault(_ x: Int = useDefault()) -> Int { x }

The actor isolation checker ensures that the isolation of default arguments matches the isolation of the callee, so this code is valid. However, default argument generators and stored property initializers don't formally model their isolation in these cases, so the compiler attempts to emit a hop in the middle of a synchronous function when the default argument itself makes a call that uses an isolated default argument value. For now, don't emit the hop if the function is synchronous. A better approach would be to formally model the isolation for these function declarations that are generated in SIL.

Resolves: rdar://124502334

[hborla]

@swift-ci please smoke test

[hborla]

@swift-ci please test Windows