[LiveDebugValues][NFC] Move LiveDebugValues source for refactor

This is a pure file move of LiveDebugValues.cpp ahead of the pass being
refactored, with an experimental new implementation to follow.

The motivation for these changes can be found here:

  http://lists.llvm.org/pipermail/llvm-dev/2020-June/142368.html

And the other related changes can be found in the phabricator stack for
this revision:

Differential Revision: https://reviews.llvm.org/D83304