Update MachOObjectFile::getSymbolAddress so it returns UnknownAddressOrSize
for undefined symbols.  Allowing llvm-nm to print spaces instead of 0’s for
the value of undefined symbols in Mach-O files.


git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@209235 91177308-0d34-0410-b5e6-96231b3b80d8