swift234

[utils] Introduce swift_build_support.diagnostics.

 - This uses more Clang/Swift-style diagnostics, and adds a convenient wrapper
   `fatal()` which happens to match what almost all clients of the previous
   `print_with_argv0` wanted to do (i.e., fail with a fatal error).

 - As part of this, I also tried to make the diagnostics more consistent and use
   "diagnostic style" casing/punctuation.

 - Part of SR-237.