As reported in PR39946, these two implementations cause stack overflows
to occur when a type recursively contains itself. While this only
happens when an incomplete version of itself is used by membership (and
thus an otherwise invalid program), the crashes might be surprising.
The solution here is to replace the recursive implementation with one
that uses a std::vector as a queue. Old values are kept around to
prevent re-checking already checked types.
Change-Id: I582bb27147104763d7daefcfee39d91f408b9fa8
llvm-svn: 348899
ef65d6c5c4