Stop modifying trailing return types.

Stop modifying trailing return types.

This change reverts the functional change from D144626 but retains its
test. Instead of dealing with the possibility that a trailing requires
clause might have been rewritten into some other incorrect form, just
stop rewriting it.

No functionality changes intended.

Reviewed By: erichkeane, ChuanqiXu

Differential Revision: https://reviews.llvm.org/D147281