[llvm] [RemoveDIs][DebugInfo] Add DPVAssign variant of DPValue (PR #77912)

[Stephen Tozer via llvm-commits]

SLTozer wrote:

This patch caused an error on some buildbots due to the line `ptrdiff_t Idx = std::distance(DebugValues.begin(), OldMD);` - the cause being that std::distance doesn't like being given an iterator and a pointer; fixed this by adding `&*` prior to the first arg to convert it to a pointer, so that we're just comparing pointers.

Submitted a fix in revision da7607307a that seems to have fixed the build on at least one affected bot.

https://github.com/llvm/llvm-project/pull/77912