[llvm] [NFC][LLVM][CodeGen] Move LiveDebugVariables.h into llvm/include/llvm/CodeGen (PR #88374)

[Piyou Chen via llvm-commits]

BeMg wrote:

> Should the PR description mention that it's needed so that passes that sit in between split register allocation can mark it as preserved? Otherwise it will get recomputed without emitting debug values (because VirtRegRewriter was invoked with ClearVirtRegs=false) which triggers an assertion when its freed

Sure. I updated the pr description.

https://github.com/llvm/llvm-project/pull/88374