[RemoveDIs][DebugInfo] Add DPValue checks to the verifier, prepare DPValue for...

[RemoveDIs][DebugInfo] Add DPValue checks to the verifier, prepare DPValue for parsing support (#79810)

As part of the RemoveDIs project, this patch adds support for checking
DPValues in the verifier. Although this is not strictly parsing-related,
and we currently automatically convert back to the old debug info format
immediately after parsing, we are approaching the point where the we can
operate end-to-end in the new debug info format, at which point it is
appropriate that we can actually validate modules in the new format.
This patch also contains some changes that aren't strictly
parsing-related, but are necessary class refactors for parsing support,
and are used in the verifier checks (i.e. changing the DILocalVariable
field to be a tracking MD reference, and adding a Verifier check to
confirm that it is a DILocalVariable).