DebugInfo: Avoid duplicating types that may be created during the process of creating their context

A partner to r188639, this is a somewhat heavy-handed fix to the general
issue, since even after that prior change the issue does still
unavoidably arise with template parameters (see test case).

There are other ways we could consider addressing this (see FIXME).

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@188640 91177308-0d34-0410-b5e6-96231b3b80d8