[llvm] [Support] Add helper struct `indent` for adding indentation (PR #108966)

[Rahul Joshi via llvm-commits]

jurahul wrote:

See https://github.com/llvm/llvm-project/commit/e51d14b670feff53a1569e13325a10ed7675d320 for a change to adopt it (which I will put out for review as a follow on).

https://github.com/llvm/llvm-project/pull/108966