Produce a better error message if Swift is run on a system without clang++

Swift on Linux (and other non-Apple platforms) calls clang++ to perform linking; if it's not found, we used to get this lousy error message:

<unknown>:0: error: link command failed with exit code 127 (use -v to see invocation)

We should do better. (It's possible we already do do better; I just don't have access to a Linux system without clang++ installed at the moment. If you have such a system, you can help out with this bug by downloading Swift from https://swift.org/download/ and seeing what echo 'print("hello")' | swiftc - produces.)