Apply patch from upstream libc++ bug 18735: http://llvm.org/bugs/show_bug.cgi?id=18735 to fix self-assignment operators of std::map, multimap and unordered_map.
Add test for std::map self-assign.
Add a note to readme.txt for libcxx to remind about backported std::map self-assign fix.
Bump version to 1.22.2 to ensure the previous patch is applied.