To catch bugs like the one fixed in

http://llvm.org/viewvc/llvm-project?view=rev&revision=78127, I'm changing the
ExecutionEngine's global mappings to hold AssertingVH<const GlobalValue>. That
way, if unregistering a mapping fails to actually unregister it, we'll get an
assert. Running the jit nightly tests didn't uncover any actual instances of
the problem.

This also uncovered the fact that AssertingVH<const X> didn't work, so I fixed
that too.

llvm-svn: 78400