[DebugInfo][NFC] Rename is/setUndef to is/setKilllocation

These names better reflect the semantics and also the implementation, since
it's not just "undef" operands that are sentinels used to signal that the debug
intrinsic terminates dominating locations definitions.

Related to https://discourse.llvm.org/t/auto-undef-debug-uses-of-a-deleted-value

Reviewed By: StephenTozer

Differential Revision: https://reviews.llvm.org/D140903