swift234

Initial support for an integrated frontend in swift_driver.

- Added support for invoking the Swift frontend via "swift_driver -frontend".
- Added frontend_main.cpp, which implements the main entry point for the
  integrated frontend. (Currently, this supports compiling an input Swift file
  into an object file.)
- Removed lib/Frontend/FrontendOptions.td, and replaced its functionality with
  options in include/Swift/Driver/Options.td and a new
  include/Swift/Driver/FrontendOptions.td. Options supported by the frontend
  are denoted by the FrontendOption flag; options which are not supported by
  the driver are denoted by the NoDriverOption flag.
- Updated CompilerInvocation::parseArgs() to use the option table returned from
  createDriverOptTable(), including renaming a handful of options. (-triple is
  now -target, and -Xclang is now -Xcc.)

Swift SVN r11082