We need to be able to locate `swift-backtrace` relative to the current
location of the runtime library.
This needs to work:
* In a Swift build directory.
* On Darwin, where we're installed in /usr/lib/swift and /usr/libexec/swift.
* On Linux, where we're in /usr/lib/swift/linux and /usr/libexec/swift/linux.
* On Windows, where we may be in a flat directory layout (because of limitations
of Windows DLL lookups).
rdar://103071801