In a Mach-O object file a relocatable expression of the form
SymbolA - SymbolB + constant is allowed when both symbols are
defined in a section. But when either symbol is undefined it
is an error.
The code was crashing when it had an undefined symbol in this case.
And should have printed a error message using the location information
in the relocation entry.
rdar://18678402
llvm-svn: 220599
9e33867d25