[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