swift234

[driver] Implemented driver-level support for immediate mode and the REPL.

Introduced OutputInfo::Mode and OutputInfo::CompilerMode, which dictate how the
driver should invoke the integrated frontend. Currently, there are three
options:

- StandardCompile: invoke the frontend once-per-input, using -primary-file
- REPL: invoke the frontend as the REPL
- Immediate: invoke the frontend in immediate mode

Updated buildOutputInfo() to properly set the CompilerMode, and updated
buildActions() and Swift::constructJob() to honor the CompilerMode.

With these changes, "swift_driver" (with no inputs) and "swift_driver -repl"
should invoke the REPL, and "swift_driver -i" should invoke the frontend in
immediate mode.

Swift SVN r12547