This reverts commit 95d94a67755620c0a2871ac6f056ca8e9731d5e9.
This implements the deferred concepts instantiation, which should allow
the libstdc++ ranges to properly compile, and for the CRTP to work for
constrained functions.
Since the last attempt, this has fixed the issues from @wlei and
@mordante.
Differential Revision: https://reviews.llvm.org/D126907
