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/func.c

@@ -84,6 +84,8 @@ FuncDeclaration::FuncDeclaration(Loc loc, Loc endloc, Identifier *id, enum STC s
     isArrayOp = false;
     allowInlining = false;
 
+    availableExternally = true; // assume this unless proven otherwise
+
     // function types in ldc don't merge if the context parameter differs
     // so we actually don't care about the function declaration, but only
     // what kind of context parameter it has.
@@ -674,6 +676,10 @@ void FuncDeclaration::semantic3(Scope *sc)
 	return;
     semanticRun = 3;
 
+    // LDC
+    if (!global.params.useAvailableExternally)
+        availableExternally = false;
+
     if (!type || type->ty != Tfunction)
 	return;
     f = (TypeFunction *)(type);