swift234

Runtime/stdlib: Slightly better implementations of errorInMain and unexpectedError.

Implement these backstops in the stdlib instead of the runtime, where we can at least interpolate the error value and give a slightly more informative error. Could still be improved by handing the source location down from the point of the crash instead of using the location from the stdlib source. Somewhat addresses rdar://problem/20807523.

Swift SVN r29401