To catch bugs like the one fixed in

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.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@78400 91177308-0d34-0410-b5e6-96231b3b80d8