[libcxx] Add logic to probe compiler in tests.

Summary:
This patch probes the cxx compiler used during testing by getting it to dump its predefined macros. Based on the value of these macros the compiler name and compiler name + version are added to the available features.

There are three compiler names:
- `clang`
- `apple-clang`
- `gcc`.

The available features added are equivalent to:
- `'%s' % compiler_name`
- `'%s-%s.%s' % compiler_name, major_version, minor_version`

This information can be used to XFAIL tests on different compilers / versions.



Reviewers: mclow.lists, danalbert, jroelofs

Reviewed By: jroelofs

Subscribers: cfe-commits

Differential Revision: http://reviews.llvm.org/D6399