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.

--- a/gen/optimizer.h	Sun Jun 07 15:07:29 2009 +0200
+++ b/gen/optimizer.h	Sun Jun 07 16:00:13 2009 +0200
@@ -5,7 +5,10 @@
 
 bool ldc_optimize_module(llvm::Module* m);
 
+// Determines whether the inliner will run in the -O<N> list of passes
 bool doInline();
+// Determines whether the inliner will be run at all.
+bool willInline();
 
 int optLevel();