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

[Luke Lau via llvm-commits]

lukel97 wrote:

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

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