[libcxx] Add code coverage configuration to CMake and LIT.

Summary:
This patch adds configuration to CMake and LIT for running the libc++ test-suite to generate code coverage.

To use code coverage use following instructions.

* Find the clang resource dir using `$CXX -print-search-dirs`. Let <library-dir> be the first library search directory.
* `cmake <regular-options> -DLIBCXX_GENERATE_COVERAGE=ON -DLIBCXX_COVERAGE_LIBRARY=<library-dir>/lib/<platform>/libclang_rt.profile.a <source>`
* `make cxx`
* `make check-libcxx`
* `make generate-libcxx-coverage`


The reason I want this patch upstreamed is so I can setup a bot that generates code coverage and posts in online for every revision. 



Reviewers: mclow.lists, jroelofs, danalbert

Reviewed By: danalbert

Differential Revision: http://reviews.llvm.org/D8716

llvm-svn: 233669