Implement the functions: clz, clzl, clzll, ctz, ctzl, and ctzll
for libcxx when compiled with Visual C++ on Win32 and Win64.

clang and gcc (MinGW) compilers provide these implementations themselves.



git-svn-id: https://llvm.org/svn/llvm-project/libcxx/trunk@198481 91177308-0d34-0410-b5e6-96231b3b80d8