[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: https://llvm.org/svn/llvm-project/libcxx/trunk@174440 91177308-0d34-0410-b5e6-96231b3b80d8
[tests] Add an available feature that combines the triple and use_system_lib.

  - This is so that we can easily write XFAIL markers for tests that are known
    to fail with versions of libc++ as were shipped with a particular triple.

git-svn-id: https://llvm.org/svn/llvm-project/libcxx/trunk@174443 91177308-0d34-0410-b5e6-96231b3b80d8
[tests] Mark some istream.unformatted expected failures (with system libc++).

git-svn-id: https://llvm.org/svn/llvm-project/libcxx/trunk@174444 91177308-0d34-0410-b5e6-96231b3b80d8