Provide ways to get back to the original Clang declaration from

a SILFunction or SILGLobalVariable.

This will be used as one step towards moving IRGen off of the
global external-definitions list.

include/swift/SIL/SILFunction.h

@@ -105,6 +105,9 @@ private:
   /// The declcontext of this function.
   DeclContext *DeclCtx;
 
+  /// The owning declaration of this function's clang node, if applicable.
+  ValueDecl *ClangNodeOwner = nullptr;
+
   /// The source location and scope of the function.
   const SILDebugScope *DebugScope;
 
@@ -146,9 +149,6 @@ private:
   /// it public.
   unsigned KeepAsPublic : 1;
 
-  /// This flag indicates if a function has a body generated by Clang.
-  unsigned ForeignBody : 1;
-
   /// This is the number of uses of this SILFunction inside the SIL.
   /// It does not include references from debug scopes.
   unsigned RefCount = 0;
@@ -515,9 +515,37 @@ public:
   bool isKeepAsPublic() const { return KeepAsPublic; }
   void setKeepAsPublic(bool keep) { KeepAsPublic = keep; }
 
-  /// Get this function's foreign body attribute.
-  HasForeignBody_t hasForeignBody() const { return HasForeignBody_t(ForeignBody); }
-  void setForeignBody(HasForeignBody_t foreign) { ForeignBody = foreign; }
+  /// Return whether this function has a foreign implementation which can
+  /// be emitted on demand.
+  bool hasForeignBody() const;
+
+  /// Return whether this function corresponds to a Clang node.
+  bool hasClangNode() const {
+    return ClangNodeOwner != nullptr;
+  }
+
+  /// Set the owning declaration of the Clang node associated with this
+  /// function.  We have to store an owner (a Swift declaration) instead of
+  /// directly referencing the original declaration due to current
+  /// limitations in the serializer.
+  void setClangNodeOwner(ValueDecl *owner) {
+    assert(owner->hasClangNode());
+    ClangNodeOwner = owner;
+  }
+
+  /// Return the owning declaration of the Clang node associated with this
+  /// function.  This should only be used for serialization.
+  ValueDecl *getClangNodeOwner() const {
+    return ClangNodeOwner;
+  }
+
+  /// Return the Clang node associated with this function if it has one.
+  ClangNode getClangNode() const {
+    return (ClangNodeOwner ? ClangNodeOwner->getClangNode() : ClangNode());
+  }
+  const clang::Decl *getClangDecl() const {
+    return (ClangNodeOwner ? ClangNodeOwner->getClangDecl() : nullptr);
+  }
 
   /// Retrieve the generic parameter list containing the contextual archetypes
   /// of the function.