Revert "Drop qualifiers from return types in C (DR423)"

Revert "Drop qualifiers from return types in C (DR423)"

This reverts commit d374b65f2da1bdd3d9a7e9ac8ed4ad5467c882f9.

The changes lose AST fidelity (reported in #55778), but also may be
improperly dropping _Atomic qualifiers. I am rolling the changes back
until I've finished discussions in WG14 about the proper resolution to
DR423.