Remove unused getMDNodeFwdRefOrNull interfaces (NFC)

Summary:
Followup from D53596/r346891. Remove the getMDNodeFwdRefOrNull interface
to the MDLoader since it is no longer used. Also improve error messages
when the internal implementation is used within the MDLoader.

Reviewers: steven_wu

Subscribers: llvm-commits

Differential Revision: https://reviews.llvm.org/D54542

llvm-svn: 346899