[NFC][Metadata] Refactor allocation, initalization and deletion of MDNodes.

This patch is refactoring the allocation, initialization and deletion
of MDNodes. It is intended as a preparatory patch for the upcoming
addition of dynamic resizability of MDNodes. It is fundamentally NFC,
but removes the necessity for suppressing the memory sanitizer for
MDNode's operator delete.

Reviewers: dexonsmith

Differential Revision: https://reviews.llvm.org/D125489