[Support] Add decodeULEB128AndInc/decodeSLEB128AndInc

Many decodeULEB128/decodeSLEB128 users need to increment the pointer.
Add helpers to simplify this common pattern. We don't add `end` and
`error` parameters at present because many users don't need them.

Pull Request: https://github.com/llvm/llvm-project/pull/85739