Added formatters for libc++ (http://libcxx.llvm.org):

 std::string has a summary provider
 std::vector std::list and std::map have both a summary and a synthetic children provider
Given the usage of a custom namespace (std::__1::classname) for the implementation of libc++, we keep both libstdcpp and libc++ formatters enabled at the same time since that raises no conflicts and enabled for seamless transition between the two
The formatters for libc++ reside in a libcxx category, and are loaded from libcxx.py (to be found in examples/synthetic)

The formatters-stl test cases have been divided to be separate for libcxx and libstdcpp. This separation is necessary because
 (a) we need different compiler flags for libc++ than for libstdcpp
 (b) libc++ inlines a lot more than libstdcpp and some code changes were required to accommodate this difference

llvm-svn: 152570