embedded: In IRGen, don't force generate code for functions which are imported from other modules.

Only generate code lazily for such functions, i.e. only if such a function is referenced from already generated code.
This is achieved by converting the SILLinkage for `public_external` functions to `internal` instead of `public in IRGenPrepare.

Author: eeckstein

lib/SILOptimizer/Mandatory/IRGenPrepare.cpp

@@ -92,7 +92,7 @@ class IRGenPrepare : public SILFunctionTransform {
       // Even de-serialized functions must be code-gen'd.
       SILLinkage linkage = F->getLinkage();
       if (isAvailableExternally(linkage)) {
-        F->setLinkage(stripExternalFromLinkage(linkage));
+        F->setLinkage(SILLinkage::Hidden);
       }
     }