This is my Driver refactoring patch.

The major changes are:
1) LinkerOptions has been merged into TargetInfo
2) LinkerInvocation has been merged into Driver
3) Drivers no longer convert arguments into an intermediate (core) argument
   list, but instead create a TargetInfo object and call setter methods on
   it. This is only how in-process linking would work. That is, you can
   programmatically set up a TargetInfo object which controls the linking.
4) Lots of tweaks to test suite to work with driver changes
5) Add the DarwinDriver
6) I heavily doxygen commented TargetInfo.h

Things to do after this patch is committed:
a) Consider renaming TargetInfo, given its new roll.
b) Consider pulling the list of input files out of TargetInfo. This will
   enable in-process clients to create one TargetInfo the re-use it with
   different input file lists.
c) Work out a way for Drivers to format the warnings and error done in
   core linking.