[NFC][FnAttrs] Stress tests for attribute deduction
This commit is a preparation of upcoming patches on attribute deduction.
It will shorten the diffs and make it clear what we inferred before.
Reviewers: chandlerc, homerdin, hfinkel, fedor.sergeev, sanjoy, spatel, nlopes
Subscribers: bollu, llvm-commits
Differential Revision: https://reviews.llvm.org/D59903