Adds classKind information to the front-end AST, which in turn allows us
to fix code generation of type names for extern(C) and extern(C++)
structs and classes.  Inspecting such types inside a debugger now just
works without the need to 'cast(module_name.cxx_type)'.

gcc/d/ChangeLog:

	* d-codegen.cc (d_decl_context): Don't include module in the name of
	class and struct types that aren't extern(D).