[tests] Add support for REQUIRES and XFAIL lines in libc++ tests.

 - We parse up to the first non-empty non-comment (C++ style) line, otherwise
   the format and semantics match what is used for LLVM/Clang tests.

 - For now, the only interesting thing to test against is a user supplied
   target_triple test parameter.

llvm-svn: 174440