[Concepts] Constraint Satisfaction Caching
Add a simple cache for constraint satisfaction results. Whether or not this simple caching
would be permitted in final C++2a is currently being discussed but it is required for
acceptable performance so we use it in the meantime, with the possibility of adding some
cache invalidation mechanisms later.
Differential Revision: https://reviews.llvm.org/D72552
(cherry picked from commit b933d37cd3774e5431b35e82187eebb59b1ff59e)