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

[Eric Fiselier]

>>! In D6255#6, @danalbert wrote:
> Should this be a feature of LIT proper? `$ lit --config path/to/lit.cfg test/dir`

I don't think it can be for a couple of reasons.

First, the way tests are discovered does not seem to allow the config file to be specified manually. I tried implementing it but it just doesn't seem to fit. The reason this works is because the lit.site.cfg file is found naturally through lit's test discovery.

Second, LIT can be invoked on multiple test suites at once. The lit.site.cfg file is specific to a single suite. I don't see how you could specify the configuration file unambiguously. For example:

```
lit -sv --config path/to/config/lit.site.cfg path/to/libcxx/test/ path/to/clang/test/
```

I hope that cleared things up.

http://reviews.llvm.org/D6255