Be consistent when deciding if a relocation is needed.

Before when deciding if we needed a relocation in A-B, we wore only checking
if A was weak.

This fixes the asymmetry.

The "InSet" argument should probably be renamed to "ForValue", since InSet is
very MachO specific, but doing so in this patch would make it hard to read.

This fixes PR22815.

llvm-svn: 234165