[PM] Defend against getting slightly wrong template arguments passed

into CRTP base classes.

This can sometimes happen and not cause an immediate failure when the
derived class is, itself, a template. You can end up essentially calling
methods on the wrong derived type but a type where many things will
appear to "work".

To fail fast and with a clear error message we can use a static_assert,
but we have to stash that static_assert inside a method body or nested
type that won't need to be completed while building the base class. I've
tried to pick a reasonably small number of places that seemed like they
would definitely get triggered on use.

This is the last of the patch series defending against this that I have
planned, so far no bugs other than the original were found.

llvm-svn: 294275

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