Issue Description:

I was writing a Swift program in Xcode 12 (Swift 5.3), and I started consistently crashing the compiler. I made a copy of the project and started removing pieces until I found what I believe is a fairly minimal example.

I'll prefix this by saying: I know this isn't a valid Swift program. I was adding conformance to a protocol, and I couldn't remember the protocol definition, so I was going to declare conformance and have Xcode show the error with the "Add stubs?" fixer. But it just kept segfaulting.

In file A.swift:

class E {
    func n(_ num: N) { }
}

func test() {
    let e = E()
    e.n(5)
}

In file B.swift:

struct N {
    let str: String

    init(_ int: Int) {
        self.str = String(int)
    }
}

extension N: ExpressibleByIntegerLiteral {

}

Curiously, Xcode reports "Command CompileSwift failed with a nonzero exit code", but changing almost any remaining aspect of this program (for example, commenting out the "self.str = String(int)" line, or the "e.n(5)" line, or even putting all the code in one file) changes the error to "Type 'N' does not conform to protocol 'ExpressibleByIntegerLiteral'" – which is what I expected in the first place.