Fixup gtest layout, add Linux ThreadStateCoordinator.cpp.

This change does the following:
* Remove test/c++/...
* Add gtest.
* Add gtest/unittest directory for unittesting individual classes.
* Add an initial Plugins/Process?linux/ThreadStateCoordinatorTest.cpp.
  - currently failing a test (intentional).
  - added a bare-bones ThreadStateCoordinator.cpp to Plugins/Process/Linux,
    more soon. Just enough to prove out running gtest on Ubuntu and MacOSX.
* Added recursive make machinery so that doing a 'make' in gtest/ is
  sufficient to kick off the existing test several directories down.
  - Caveat - I currently short circuit from gtest/unittest/Makefile directly to
    the one and only gtest/unittest/Plugins/Process/Linux directory.  We'll need
    to add the intervening layers.  I haven't done this yet since to fix the
    Xcode test failure correspondence, I may need to add a python layer which
    might just handle the directory crawling.
* Added an Xcode project to the lldb workspace for gtest.
  - Runs the recursive make system in gtest/Makefile.
  - Default target is 'test'.  test and clean are supported.
  - Currently does not support test failure file/line correspondence.
    Requires a bit of text transformation to hook that up.




git-svn-id: https://llvm.org/svn/llvm-project/lldb/trunk@218460 91177308-0d34-0410-b5e6-96231b3b80d8