10532: Rename `descend_into_macros` Function per FIXME comment r=Veykril a=mirkoRainer This renames `descend_into_macros` to `descend_into_macros_single` and `descend_into_macros_many` into `descend_into_macros`. However, this does not touch a function in `SemanticsImpl` of same name. I was prompted to do this per a FIXME comment, which is removed in this PR. Co-authored-by: Mirko Rainer <mirkorainer@outlook.com>