Run semantic3 on imported modules, and emit new symbols with

`available_externally` linkage. This allows the inliner to inline functions from
other modules while telling the code generator to ignore those functions (treat
them as declarations)
Still generates a few extra `TypeInfo`s and strings...
Disabled when generating debug info because I don't really understand it, and it
doesn't like this.

gen/classes.cpp

@@ -491,7 +491,9 @@ LLValue* DtoIndexClass(LLValue* src, ClassDeclaration* cd, VarDeclaration* vd)
     {
         Logger::cout() << "src2: " << *src << '\n';
         Logger::cout() << "index: " << field->index << '\n';
+#if 0
         Logger::cout() << "srctype: " << *src->getType() << '\n';
+#endif
     }
 #endif
     LLValue* val = DtoGEPi(src, 0, field->index);