Stabilize and simplify SIL linkage and serialization

The main point of this change is to make sure that a shared function always has a body: both, in the optimizer pipeline and in the swiftmodule file.
This is important because the compiler always needs to emit code for a shared function. Shared functions cannot be referenced from outside the module.
In several corner cases we missed to maintain this invariant which resulted in unresolved-symbol linker errors.

As side-effect of this change we can drop the shared_external SIL linkage and the IsSerializable flag, which simplifies the serialization and linkage concept.

lib/SILOptimizer/Transforms/StringOptimization.cpp

@@ -693,7 +693,7 @@ ApplyInst *StringOptimization::createStringInit(StringRef str,
       return nullptr;
     
     auto Mangled = SILDeclRef(makeUTF8Decl, SILDeclRef::Kind::Allocator).mangle();
-    makeUTF8Func = module.findFunction(Mangled, SILLinkage::PublicExternal);
+    makeUTF8Func = module.loadFunction(Mangled, SILModule::LinkingMode::LinkAll);
     if (!makeUTF8Func)
       return nullptr;
   }