Issue Description:

I'm trying to create and use a SyntaxRewriter subclass from a thread other than main. I'm not sharing between threads, so it seems like it should be fine. However, I get an EXEC_BAD_ACCESS inside of SwiftSyntax. Here's some sample code that demonstrates the problem, which is adapted from the SwiftSyntax README's sample code:

let string = """

protocol P {
    var x: Int? { get set }
}


"""

let queue = DispatchQueue.global()
let sourceFile = try! SyntaxParser.parse(source: string)
let semaphore = DispatchSemaphore(value: 0)
queue.async {
    _ = Renamer().visit(sourceFile)
    semaphore.signal()
}
semaphore.wait()
print("done")

It crashes with an EXEC_BAD_ACCESS, typically in `SyntaxRewriter.visitChildren()`. Trying to print any variables in the debugger outputs "Trying to put the stack in unreadable memory at <some memory address>".

I can't reproduce this in an optimized build.