Exclude 2 codegen-related files from dub frontend subpackage

These import backend modules, and that leads to undefined symbol
regressions (backend TypeInfos referencing undefined backend init
symbols) with LDC on Windows for current master (stable is fine).
See https://github.com/ldc-developers/ldc/issues/3212.

dub.sdl

@@ -83,10 +83,12 @@ subPackage {
     iasm,\
     iasmdmd,\
     iasmgcc,\
+    irstate,\
     objc_glue,\
     s2ir,\
     tocsym,\
     toctype,\
+    tocvdebug,\
     toobj,\
     todt,\
     toir\