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/dmd/mars.h	Sun Jun 07 15:07:29 2009 +0200
+++ b/dmd/mars.h	Sun Jun 07 16:00:13 2009 +0200
@@ -237,6 +237,7 @@
     bool llvmAnnotate;
     bool useInlineAsm;
     bool verbose_cg;
+    bool useAvailableExternally;
 
     // target stuff
     const char* llvmArch;