Xcode: added lldb-gtest target to build and run the gtests in the unittests directory.

Also added a placeholder Editline gtest for some code that I'll add as soon
as I make sure this addition doesn't break any of the build bots.

This change also introduces some Xcode user-defined variables that I've used
to attempt to isolate the way Python is integrated into the build.  I don't have
the rest of LLDB using it yet, I'm using the gtests as my guinea pig on that.
Currently these are:
PYTHON_FRAMEWORK_PATH
PYTHON_VERSION_MAJOR
PYTHON_VERSION_MINOR

I will convert the rest over to it after this gets a little time to bake
and any kinks are worked out of it.

llvm-svn: 251261