We already try using `CFStringGetCStringPtr` to see if we can get ASCII;
we should also try `CFStringGetCharactersPtr` as the native encoding of
`CFNSString` is very often UTF-16. While we're at it, try asking explicitly
for UTF-8 as well. That won't work for `CFNSString` because AFAIK it's
currently never encoded that way, but it might work if something hands us
a Swift-backed string or if some future implementation of `CFString`
happens to support UTF-8 encoded data.
rdar://124913332
Move the backtracing code into a new Runtime module. This means renaming
the Swift Runtime's CMake target because otherwise there will be a name
clash.
rdar://124913332
