[llvm] [NFC] Add `const` (PR #116611)

[Han-Kuan Chen via llvm-commits]

HanKuanChen wrote:

Maybe you need to separate the commit? One for adding const for member functions and the other one for adding const for pointers.

The `copyFrom` for `llvm/include/llvm/CodeGen/LiveInterval.h` is irrelevant to const.

https://github.com/llvm/llvm-project/pull/116611