Fix iterator invalidation. PR16935.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@188835 91177308-0d34-0410-b5e6-96231b3b80d8
Test case for PR16933/r188707.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@188836 91177308-0d34-0410-b5e6-96231b3b80d8