This injects a customization point for hashtable-specific equality
testing that the key context uses by default. While this is rarely
needed, there are LLVM types where it is necessary and it seems a good
general tool to have to avoid unnecessary complexity from custom key
contexts when a simple customization of equality is all that is
required.
This also adds a CRTP mixin for implementing a common pattern of key
contexts where the context provides translation of some key types into
another type, potentially using state. Rather than having to implement
the entire key context API, code can derive from this template and
simply provide a set of overloads for the types it wants to translate.
Any key types used which can be passed to one of those overloads will
get translated before following the same logic as the default key
context. While this updates the only usage so far of this pattern, a
subsequent PR will add several more users making the pattern worth
abstracting here.
