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

git-svn-id: 273b07cebdae5845b4a75aae04570b978db9eb50@174440 91177308-0d34-0410-b5e6-96231b3b80d8

lodyagin/bare_cxx,lodyagin/bare_cxx,lodyagin/bare_cxx,lodyagin/bare_cxx,lodyagin/bare_cxx