[Sema][ObjC] Do not propagate the nullability specifier on the receiver
to the result type of a message send if the result type cannot have a
nullability specifier.

Previously, clang would print the following message when the code in
nullability.m was compiled:

"incompatible integer to pointer conversion initializing 'int *' with
an expression of type 'int _Nullable'"

This is wrong as 'int' isn't supposed to have any nullability
specifiers.

rdar://problem/40830514

llvm-svn: 338048
diff --git a/clang/lib/Sema/SemaExprObjC.cpp b/clang/lib/Sema/SemaExprObjC.cpp
index bf9d081..bf0ffeb 100644
--- a/clang/lib/Sema/SemaExprObjC.cpp
+++ b/clang/lib/Sema/SemaExprObjC.cpp
@@ -1357,6 +1357,11 @@
   if (isClassMessage)
     return resultType;
 
+  // There is nothing left to do if the result type cannot have a nullability
+  // specifier.
+  if (!resultType->canHaveNullability())
+    return resultType;
+
   // Map the nullability of the result into a table index.
   unsigned receiverNullabilityIdx = 0;
   if (auto nullability = ReceiverType->getNullability(Context))