Change foldl' to inline when partially applied (#19534)

WIP because I'm not sure about adding test cases (it shouldn't change any behaviour, but it should cause more inlining and I'm not sure how to test that) and I want to make sure it passes the CI first.

• are either individually buildable or squashed
• have commit messages which describe what they do (referring to Notes and tickets using #NNNN syntax when appropriate)
• have added source comments describing your change. For larger changes you likely should add a Note and cross-reference it from the relevant places.
• add a testcase to the testsuite.
• replace this message with a description motivating your change

If you have any questions don't hesitate to open your merge request and inquire in a comment. If your patch isn't quite done yet please do add prefix your MR title with WIP:.