[codeview] Extend the heuristic for detecting classes imported from DLLs

If a dynamic class contains a dllimport method, then assume the class
may not be constructed in this DLL, and therefore the vtable will live
in a different PDB.

This heuristic is still incomplete, and will miss things like abstract
base classes that are only constructed on one side of the DLL interface.
That said, this heuristic does detect some cases that are currently
problematic, and may be useful to other projects that don't use many
DLLs.

llvm-svn: 281053