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Issue Description:

When accessing a constant member of an actor defined within another module, the Swift compiler crashes.

The attached project contains two modules: "dependency" in which an actor "A" with a constant member is declared, and "SwiftCrash" which depends on "dependency" and uses "A" within an async function.

Expected behaviour

The expected declaration of said function:

func aFunction(a: A) -> String{    
    return a.requiredString
} 

Which compiles as long as "A" is defined within the same module.

When "A" is defined within another module, the compiler will throw one of the following errors depending on whether "aFunction" is async:

Sync:

actor-isolated property 'requiredString' can not be referenced from a non-isolated context 

Async:

Expression is 'async' but is not marked with 'await'