embedded: allow dead function elimination for de-serialized functions

In embedded swift all de-serialized get public linkage because all the code is generated in the top-level module.
This change moves the point where we make de-serialized functions public to the end of the pipeline.
This allows dead function elimination to remove unused de-serialized functions.
For some stdlib functions (actually one: the Double initializer for a builtin integer) is essential, because codegen for embedded produces an unresolved symbol.

rdar://123772098

lib/SIL/IR/SILFunction.cpp

@@ -940,6 +940,9 @@ bool SILFunction::shouldBePreservedForDebugger() const {
   if (getEffectiveOptimizationMode() != OptimizationMode::NoOptimization)
     return false;
 
+  if (isAvailableExternally())
+    return false;
+
   if (hasSemanticsAttr("no.preserve.debugger"))
     return false;