Fix bitcode desynchronization when loading a PCH containing a class template
specialization with modules enabled. Just don't merge them at all for now;
we'll revisit this when support for template merging is added.

In passing, make Decl::dump() a little safer to use with PCH/modules, by making
it not deserialize any additional declarations. From a debugger you can call
decls_begin() or similar first if you want to dump all child decls.