WG14 DR423 (https://www.open-std.org/jtc1/sc22/wg14/www/docs/n2148.htm#dr_423), resolved during the C11 time frame, changed the way qualifiers are handled on function return types and in cast expressions after it was noted that these types are now directly observable via generic selection expressions. In C, the function declarator is adjusted to ignore all qualifiers (including _Atomic qualifiers).
Clang already handles the cast expression case correctly (by performing the lvalue conversion, which drops the qualifiers as well), but with these changes it will now also handle function declarations appropriately.
Fixes #39595
Hrmph, this is unfortunate...