Fix a crash with X86 Mach-O and a subtraction expression where both symbols are
undefined and produce an error message instead as this is a non-relocatable
expression with X86 Mach-O.

rdar://8920876


git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@188218 91177308-0d34-0410-b5e6-96231b3b80d8