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/SIL/IR/SILFunctionBuilder.cpp

@@ -249,7 +249,7 @@ SILFunction *SILFunctionBuilder::getOrCreateFunction(
            (forDefinition == ForDefinition_t::NotForDefinition &&
             (fnLinkage == linkageForDef ||
              (linkageForDef == SILLinkage::PublicNonABI &&
-              fnLinkage == SILLinkage::SharedExternal))));
+              fnLinkage == SILLinkage::Shared))));
     if (forDefinition) {
       // In all the cases where getConstantLinkage returns something
       // different for ForDefinition, it returns an available-externally