[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

git-svn-id: https://llvm.org/svn/llvm-project/libcxx/trunk@223593 91177308-0d34-0410-b5e6-96231b3b80d8
Unify and cleanup rpath handling in tests.

git-svn-id: https://llvm.org/svn/llvm-project/libcxx/trunk@223594 91177308-0d34-0410-b5e6-96231b3b80d8
Merge branch 'master' of http://llvm.org/git/libcxx
Fix apple clang detection in lit.cfg

git-svn-id: https://llvm.org/svn/llvm-project/libcxx/trunk@223597 91177308-0d34-0410-b5e6-96231b3b80d8
Merge branch 'master' of http://llvm.org/git/libcxx
Revert parts of r223594. Use DYLD_LIBRARY_PATH on OSX when running tests.

git-svn-id: https://llvm.org/svn/llvm-project/libcxx/trunk@223598 91177308-0d34-0410-b5e6-96231b3b80d8
Merge branch 'master' of http://llvm.org/git/libcxx
Consolidate error reporting in lit.cfg

git-svn-id: https://llvm.org/svn/llvm-project/libcxx/trunk@223599 91177308-0d34-0410-b5e6-96231b3b80d8
Merge branch 'master' of http://llvm.org/git/libcxx
Mark a couple of tests as XFAIL with older compilers.

git-svn-id: https://llvm.org/svn/llvm-project/libcxx/trunk@223600 91177308-0d34-0410-b5e6-96231b3b80d8
Merge branch 'master' of http://llvm.org/git/libcxx