Reland [Metadata] Add a resize capability to MDNodes and add a push_back interface to MDNode

Fixed a bug with double destruction of operands and corrected a test issue.

Note that this patch leads to a slight increase in compile time (I measured
about .3%) and a slight increase in memory usage. The increased memory usage
should be offset once resizing is used to a larger extent.

Reviewed By: dexonsmith

Differential Revision: https://reviews.llvm.org/D125998