added Xcode lldb-gtest-for-debugging target

The lldb-gtest target is for CI and runs the tests as
part of the build phase.  It does not support debugging
the gtests from Xcode, though, due to the run happening
during the build phase.

This change adds a lldb-gtest-for-debugging target that
can be used to debug gtests.

llvm-svn: 279354