[LiveDebugValues] Remove PendingInLocs (NFC)

PendingInLocs ends up having the same value as InLocs, just computed
a bit more indirectly. It is a leftover of a previous implementation
approach.

This patch drops PendingInLocs, as well as the Diff and Removed
calulations, which are no longer needed.

Differential Revision: https://reviews.llvm.org/D80868