Add a pass to strip the bodies of available_externally functions so string

literals and `TypeInfo`s only referenced by them can be deleted by `-globaldce`.

gen/optimizer.cpp

@@ -222,6 +222,9 @@ static void addPassesForOptLevel(PassManager& pm) {
     }
 
     if (optimizeLevel >= 1) {
+#if LLVM_REV >= 68940
+        addPass(pm, createStripExternalsPass());
+#endif
         addPass(pm, createGlobalDCEPass());
     }