Fix an issue with finding python on Windows.

Someone must have changed the behavior of FileSpec slightly
relating to whether or not there is a trailing backslash when calling
GetPath() and GetDirectory().  This caused ScriptInterpreterPython
to find the wrong values when initializing sys.path, and as a result
we couldn't find the lldb module.

This patch fixes the issue, and also adds a test to make sure that
GetDirectory() does not return a string containing a trailing slash.

llvm-svn: 237282