[ADT] Generalize pointee_iterator to smart pointers by using decltype.
Based on review feedback from Dave on the original patch.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@207146 91177308-0d34-0410-b5e6-96231b3b80d8
[LCG] Switch a weird do/while loop that actually couldn't fail its
condition into an obviously infinite loop with an assert about the
degenerate condition. No functionality changed.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@207147 91177308-0d34-0410-b5e6-96231b3b80d8