Add remote testing support to the lit config.

Add remote testing support to the lit config.

Executors can be specified at configure time by using the -DLIBCXX_EXECUTOR=""
option. Examples include:

  $ cmake <other_flags> -DLIBCXX_EXECUTOR="TimeoutExecutor(30,LocalExecutor())"
      This runs individual tests with a maximum duration

  $ cmake <other_flags> -DLIBCXX_EXECUTOR="SSHExecutor('hostname','username')"
      This runs tests on a remote target, using scp to shuttle binaries to the
      target, and ssh to invoke commands there.

  $ cmake <other_flags> -DLIBCXX_EXECUTOR="PrefixExecutor('/path/to/run/script',LocalExecutor())"
      This assumes the script knows how to copy run the executables passed to it,
      and allows for the ultimate control. This is useful for running things
      inside emulators like Valgrind & QEMU.

TODO: This doesn't claim to support ShTest tests yet, that will take a bit more
  thought & finagling (I'm still not sure how to orchestrate copy-in for those cases.

  I've also punted on what to do about tests that read data files. The testsuite
  has several tests that need to read *.dat files placed next to them, and
  currently those aren't copied over when using, say, an SSHExecutor. The
  affected tests are:

     libc++ :: std/input.output/file.streams/fstreams/filebuf.virtuals/pbackfail.pass.cpp
     libc++ :: std/input.output/file.streams/fstreams/filebuf.virtuals/underflow.pass.cpp
     libc++ :: std/input.output/file.streams/fstreams/ifstream.assign/member_swap.pass.cpp
     libc++ :: std/input.output/file.streams/fstreams/ifstream.assign/move_assign.pass.cpp
     libc++ :: std/input.output/file.streams/fstreams/ifstream.assign/nonmember_swap.pass.cpp
     libc++ :: std/input.output/file.streams/fstreams/ifstream.cons/move.pass.cpp
     libc++ :: std/input.output/file.streams/fstreams/ifstream.cons/pointer.pass.cpp
     libc++ :: std/input.output/file.streams/fstreams/ifstream.cons/string.pass.cpp
     libc++ :: std/input.output/file.streams/fstreams/ifstream.members/close.pass.cpp
     libc++ :: std/input.output/file.streams/fstreams/ifstream.members/open_pointer.pass.cpp
     libc++ :: std/input.output/file.streams/fstreams/ifstream.members/open_string.pass.cpp
     libc++ :: std/input.output/file.streams/fstreams/ifstream.members/rdbuf.pass.cpp
     libc++ :: std/localization/locales/locale.convenience/conversions/conversions.buffer/pbackfail.pass.cpp
     libc++ :: std/localization/locales/locale.convenience/conversions/conversions.buffer/underflow.pass.cpp

Note: One thing to watch out for when using the SSHExecutor for cross-testing is
  that you'll also want to specify a TargetInfo object (so that the host's
  features aren't used for available-features checks and flags setup).

http://reviews.llvm.org/D7380

llvm-svn: 230592