Emit debug info for dynamic classes if they are imported from a DLL.

With -debug-info-kind=limited, we omit debug info for dynamic classes that live in other TUs. This reduces duplicate type information. When statically linked, the type information comes together. But if your binary has a class derived from a base in a DLL, the base class info is not available to the debugger.

The decision is made in shouldOmitDefinition (CGDebugInfo.cpp). Per a suggestion from rnk, I've tweaked the decision so that we do include definitions for classes marked as DLL imports. This should be a relatively small number of classes, so we don't pay a large price for duplication of the type info, yet it should cover most cases on Windows.

Essentially this makes debug info for DLLs independent, but we still assume that all TUs within the same DLL will be consistently built with (or without) debug info and the debugger will be able to search across the debug info within that scope to resolve any declarations into definitions, etc.

llvm-svn: 278861