Rework the gtest directory structure.

This makes the directory structure mirror the canonical LLVM
directory structure for a gtest suite.

Additionally, this patch deletes the xcode project.  Nobody
is currently depending on this, and it would be better to have
gtest unit tests be hand-maintained in the Xcode workspace
rather than using this python test runner.  Patches to that
effect will be submitted as followups.

llvm-svn: 232211