PR14021: Copy lookup results to ensure safe iteration.

Within the body of the loop the underlying map may be modified via

  Sema::AddOverloadCandidate
    -> Sema::CompareReferenceRelationship
    -> Sema::RequireCompleteType

to avoid the use of invalid iterators the sequence is copied first.

A reliable, though large, test case is available - it will be reduced and
committed shortly.

Patch by Robert Muth. Review by myself, Nico Weber, and Rafael Espindola.

llvm-svn: 166188