remove dependence of TestGdbRemoteExitCode.py on parent directory source

As Pavel pointed out in a comment on llvm.org/pr30271, the VPATH I was
using here to eliminate duplication of a .cpp file had a side effect of
attempting to pull in a .o/.obj file from that same parent dir, where
other tests can be running in parallel.  This is no good.

For now, I have removed the VPATH, which should address
llvm.org/pr30271.  I have also removed the XFAIL.

llvm-svn: 280675