[Serialization] Fix a bug in the serialization of resilient functions

The SIL serializer can decide not to serialize the body of functions in the SIL
module, and only emit a declaration.  If we keep the original linkage kind
(public, private, etc) then the deserialized module won't pass verification
because we do not allow internal functions to have external declarations. This
commit changes the linkage kind for the functions that the serializer decides to
emit as a declaration (without a body).

rdar://21989088

Author: nadavrot

include/swift/SIL/SILLinkage.h

@@ -110,6 +110,19 @@ inline SILLinkage stripExternalFromLinkage(SILLinkage linkage) {
   return linkage;
 }
 
+/// Add the 'external' attribute to \p linkage.
+inline SILLinkage addExternalToLinkage(SILLinkage linkage) {
+  if (linkage == SILLinkage::Public)
+    return SILLinkage::PublicExternal;
+  if (linkage == SILLinkage::Hidden)
+    return SILLinkage::HiddenExternal;
+  if (linkage == SILLinkage::Shared)
+    return SILLinkage::SharedExternal;
+  if (linkage == SILLinkage::Private)
+    return SILLinkage::PrivateExternal;
+  return linkage;
+}
+
 /// Return whether the linkage indicates that an object has a
 /// definition outside the current SILModule.
 inline bool isAvailableExternally(SILLinkage linkage) {

lib/Serialization/SerializeSIL.cpp

@@ -255,14 +255,24 @@ void SILSerializer::writeSILFunction(const SILFunction &F, bool DeclOnly) {
   if (hasSharedVisibility(Linkage))
     Linkage = SILLinkage::Shared;
 
+  // Check if we need to emit a body for this function.
+  bool NoBody = DeclOnly || isAvailableExternally(Linkage) ||
+                F.isExternalDeclaration();
+
+  // If we don't emit a function body then make sure to mark the decleration
+  // as available externally.
+  if (NoBody) {
+    Linkage = addExternalToLinkage(Linkage);
+  }
+
   SILFunctionLayout::emitRecord(
       Out, ScratchRecord, abbrCode, toStableSILLinkage(Linkage),
       (unsigned)F.isTransparent(), (unsigned)F.isFragile(),
       (unsigned)F.isThunk(), (unsigned)F.isGlobalInit(),
       (unsigned)F.getInlineStrategy(), (unsigned)F.getEffectsKind(),
       FnID, SemanticsID);
 
-  if (DeclOnly || isAvailableExternally(Linkage) || F.isExternalDeclaration())
+  if (NoBody)
     return;
 
   // Write the body's context archetypes, unless we don't actually have a body.