[clangd] [HeuristicResolver] Protect against infinite recursion on DependentNameTypes (#83542)

When resolving names inside templates that implement recursive
compile-time functions (e.g. waldo<N>::type is defined in terms
of waldo<N-1>::type), HeuristicResolver could get into an infinite
recursion, specifically one where resolveDependentNameType() can
be called recursively with the same DependentNameType*.

To guard against this, HeuristicResolver tracks, for each external
call into a HeuristicResolver function, the set of DependentNameTypes
that it has seen, and bails if it sees the same DependentNameType again.

To implement this, a helper class HeuristicResolverImpl is introduced
to store state that persists for the duration of an external call into
HeuristicResolver (but does not persist between such calls).

Fixes https://github.com/clangd/clangd/issues/1951