Include unreferenced nested types in member list only for CodeView

Unreferenced nested structs and classes were omitted from the debug info.  In DWARF, this was intentional, to avoid bloat.  But for CodeView, we want this information to be consistent with what Microsoft tools would produce and expect.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@276271 91177308-0d34-0410-b5e6-96231b3b80d8