swift234

[frontend] Implemented support for emitting serialized diagnostics.

Reworked the -serialize-diagnostics option, so that it is now just a flag
indicating that the frontend should generate serialized diagnostics. The path at
which the diagnostics will be serialized is now specified by the
-serialized-diagnostics-path option, which is a frontend-only option. (The
frontend treats -serialized-diagnostics-path as implying -serialize-diagnostics.)

If -serialize-diagnostics is passed but -serialized-diagnostics-path is not
passed, the frontend will choose an output path from a few default values:

- If the frontend has a non-stdout output path, replace that path's extension
  with .dia.
- If there is a primary input filename, use that input's base name as the base
  name for the serialized diagnostics file.
- Otherwise, use the module name as the base name for the serialized diagnostics
  file.

Added support for setting up a serialized diagnostics DiagnosticConsumer in
frontend_main() if FrontendOptions::SerializedDiagnosticsPath is non-empty.

Swift SVN r12251