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/declaration.h

@@ -300,6 +300,11 @@ struct VarDeclaration : Declaration
     /// Set during type generation.
     unsigned aggrIndex;
 
+    /// Variables that wouldn't have gotten semantic3'ed if we weren't inlining set this flag.
+    bool availableExternally;
+    /// Override added to set above flag.
+    void semantic3(Scope *sc);
+
     // FIXME: we're not using these anymore!
     AnonDeclaration* anonDecl;
     unsigned offset2;
@@ -754,6 +759,9 @@ struct FuncDeclaration : Declaration
     // if this is an array operation it gets a little special attention
     bool isArrayOp;
 
+    // Functions that wouldn't have gotten semantic3'ed if we weren't inlining set this flag.
+    bool availableExternally;
+
     // true if overridden with the pragma(allow_inline); stmt
     bool allowInlining;
 #endif