[AST] Fix bug in UnresolvedSet::erase of last element

UnresolvedSet::erase works by popping the last element then replacing
the element to be erased with that element. When the element to be
erased is itself the last element this leads to writing past the end
of the set, causing an assertion failure.

Fix this by making erase of the last element just pop that element.

Differential Revision: https://reviews.llvm.org/D154502