Fix one of the regressions found in revert of concept sugaring

It seems that the sugaring patches had some pretty significant
interdependencies, and at least one issue exists that requires part of
the concept-sugaring patch.  I don't believe this to have anything to do
with the increased compile-times that were seen, so hopefully this will
fix our problems without further regressions.

See https://reviews.llvm.org/rG12cb1cb3720de8d164196010123ce1a8901d8122
for discussion.

Differential Revision: https://reviews.llvm.org/D142500