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.

dmd/struct.c

@@ -52,6 +52,10 @@ AggregateDeclaration::AggregateDeclaration(Loc loc, Identifier *id)
     ctor = NULL;
     defaultCtor = NULL;
 #endif
+
+#if IN_LLVM
+    availableExternally = true; // assume this unless proven otherwise
+#endif
 }
 
 enum PROT AggregateDeclaration::prot()
@@ -81,6 +85,10 @@ void AggregateDeclaration::semantic2(Scope *sc)
 void AggregateDeclaration::semantic3(Scope *sc)
 {   int i;
 
+    // LDC
+    if (!global.params.useAvailableExternally)
+        availableExternally = false;
+
     //printf("AggregateDeclaration::semantic3(%s)\n", toChars());
     if (members)
     {