[libcxx] Add numerous options to libc++ LIT test suite configuration.

Summary:
In order to fully replace the testit script we need to update LIT so it provides the same functionality.
This patch adds a number of different configuration options to LIT to do that. It also adds documentation for all of the command line parameters that LIT supports.

Generic options added:
- `libcxx_headers`
- `libcxx_library`
- `compile_flags`


Generic options modified:
- `link_flags`: Changed from overriding the default args to adding extra args instead (to match compile flags)
- `use_sanitizer`: Renamed from `llvm_use_sanitizer`


Please see the added documentation for more information about the switches. As for the actual documentation I'm not sure if it should be kept in libc++ forever since it adds an undue maintenance burden, but I think it should be added for the time being while the changes are new. I'm verify unskilled with HTML so if the documentation needs any changes please let me know.

Hopefully this will kill testit.



Reviewers: jroelofs, mclow.lists, danalbert

Reviewed By: danalbert

Subscribers: alexfh, cfe-commits

Differential Revision: http://reviews.llvm.org/D5877