PR21323: Fix iterator invalidation issue when looking for redeclarations of a
special member function.

No test yet: the only testcases we have for this issue are extremely complex.
Testcase will be added once I get a reasonable reduction.


git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@222506 91177308-0d34-0410-b5e6-96231b3b80d8