Don't leak default arg tokens on invalid destructors or conversion functions.

Fixes PR19689, and possibly the last leak in clang (PR19521). PR19689 contains
a few suggestions on how to make the default argument token code a bit more
solid, these changes can possibly be landed in follow-ups.


git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@208481 91177308-0d34-0410-b5e6-96231b3b80d8