Add "noreturn" attribute : stage 1

- Add the attribute to AnyFunctionType::ExtInfo.
- Propagate the attributes from DeclAttributes to AnyFunctionType for
  FuncDecls in TypeCheckDecl.cpp.
- Make sure the new attribute is serialized.

The main missing pieces are checking the applicability of the type attributes
on the FuncDecl and teaching typechecker about conversions on types with
noreturn.

Swift SVN r6359

lib/Serialization/Serialization.cpp

@@ -1097,6 +1097,8 @@ bool Serializer::writeDecl(const Decl *D) {
     attrs.Assignment = false;
     attrs.Conversion = false;
     attrs.AsmName = {};
+    attrs.NoReturn = false;
+    attrs.Thin = false;
 
     // FIXME: Handle other attributes.
     if (!attrs.empty())
@@ -1393,6 +1395,7 @@ bool Serializer::writeType(Type ty) {
                                    getRawStableCC(fnTy->getAbstractCC()),
                                    fnTy->isAutoClosure(),
                                    fnTy->isThin(),
+                                   fnTy->isNoReturn(),
                                    fnTy->isBlock());
 
     return true;
@@ -1409,7 +1412,8 @@ bool Serializer::writeType(Type ty) {
                                               addTypeRef(fnTy->getResult()),
                                               addDeclRef(genericContext),
                                               getRawStableCC(callingConvention),
-                                              fnTy->isThin());
+                                              fnTy->isThin(),
+                                              fnTy->isNoReturn());
 
     return true;
   }