Incremental ThinLTO tests attribute `rustc_expected_cgu_reuse` ignored

[Enselic]

Currently rustc_expected_cgu_reuse is in practice ignored. These tests use that attribute:

• tests/incremental/thinlto/cgu_invalidated_via_import.rs
• tests/incremental/thinlto/cgu_keeps_identical_fn.rs
• tests/incremental/thinlto/independent_cgus_dont_affect_each_other.rs

Step-by-step

1. Change the value of kind field of any rustc_expected_cgu_reuse attribute in any test that uses rustc_expected_cgu_reuse.
2. Run the test, e.g. with ./x test tests/incremental/thinlto

Expected

Test fails since we changed the expected outcome

Actual

Test pass, because rustc_expected_cgu_reuse is in practice ignored.

Example:
This PR should have failed CI but it didn't: #118971

How to fix

Here is a PR with a fix: #118973