These are by far the most common types to be parents in the AST so it makes
sense to optimize for them. Put them directly into the value of the map.
This currently saves 32 bytes per parent in the map and a pointer
indirection at the cost of some additional complexity in the code.
Sadly this means we cannot return an ArrayRef from getParents anymore, add
a proxy class that can own a single DynTypedNode and otherwise behaves
exactly the same as ArrayRef.
For example on a random large file (X86ISelLowering.cpp) this reduces the size
of the parent map by 24 MB.