Mark ptrs as `llvm.nocapture`

potentially a follow up pr, but nocapture is likely to be immensely useful too [aka, for a pointer/memref input, it isn't stored or returned]

_Originally posted by @wsmoses in https://github.com/EnzymeAD/Enzyme-JAX/pull/1439#discussion_r2396717528_

Start by reviewing the discussion in PR 1439, which identifies pointer or memref inputs as the relevant entry point. No file or test is named, so locate where LLVM attributes are emitted and determine how to verify that inputs which are not stored or returned receive llvm.nocapture.

Written by the indexing model from the issue text.