Warn when 'assume_nonnull' infers nullability within an array.

...or within a reference. Both of these add an extra level of
indirection that make us less certain that the pointer really was
supposed to be non-nullable. However, changing the default behavior
would be a breaking change, so we'll just make it a warning instead.

Part of rdar://problem/25846421

llvm-svn: 286521