[libcxx] Teach libcxx's lit configuration new ways to find lit.site.cfg

Summary:
Currently to run tests in tree you need to symlink the lit.site.cfg file generated by the cmake build into the source tree, and teach your VCS to ignore it.

This allows the user to specify where to find the lit.site.cfg file two different ways:
* lit_site_config lit parameter
* LIT_SITE_CONFIG enviroment variable.

example usage:
```
lit -sv --param=libcxx_site_config=path/to/libcxx-build/test/lit.site.cfg path/to/tests
```
Or
```
export LIBCXX_SITE_CONFIG=path/to/libcxx-build/test/lit.site.cfg
lit -sv path/to/tests
```
The command line parameter will override the environment variable.
If neither options are present a warning is issued and the `lit.cfg` file is loaded directly.


Reviewers: mclow.lists, jroelofs, danalbert

Reviewed By: danalbert

Subscribers: ddunbar, cfe-commits

Differential Revision: http://reviews.llvm.org/D6255