Refresh MAINTAINERS paths and test maintainer lookup

## Problem

The repository's `MAINTAINERS` file still contains `llvm_temporary/`, which no longer exists. Current areas such as `llvm/`, `utils/`, tooling, workflows, and Rust regression tests are either missing or fall through to the default maintainer.

The lookup script also has no automated tests to catch stale path patterns.

## Scope

- Replace stale paths with the current repository layout.
- Cover compiler frontend, LLVM backend, CLI/driver, shared utilities, standard library, tests, CI, and contributor tooling.
- Keep the current maintainer contacts unless maintainers explicitly request a change.
- Add Python unit tests for exact directory matches, nested files, multiple input paths, and unmatched files.
- Add a check that each `F:` pattern matches at least one tracked repository path.

## Completion criteria

- [ ] Every maintained top-level code area has an explicit pattern.
- [ ] No `F:` pattern points only to nonexistent paths.
- [ ] `tools/get_maintainer.py` tests run in CI.
- [ ] The CLI output remains suitable for email patch workflows.

Start by reading MAINTAINERS and tools/get_maintainer.py, then inspect the current tracked repository paths and existing CI configuration. Update the patterns while preserving maintainer contacts, add Python unit tests for exact, nested, multiple, and unmatched paths, and verify every F: pattern matches a tracked path and the tests run in CI.

Written by the indexing model from the issue text.