Make PR20038.cpp test case portable to non-Itanium ABIs

The only call in this function is to the dtor, so there's no need to
name a non-portable mangled function name to match it.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@211742 91177308-0d34-0410-b5e6-96231b3b80d8