Remember if a weakref of a symbol has been used.

Remember if a weakref of a symbol has been used.

This avoids yet another last minute patching of the binding.

While at it, also simplify the weakref implementation a bit by not walking
past it in the expression evaluation.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@238982 91177308-0d34-0410-b5e6-96231b3b80d8