Add support for the notion of "hidden" relocations. On MachO, these are...

Add support for the notion of "hidden" relocations.  On MachO, these are relocation entries that are used as additional information for other, real relocations, rather than being relocations themselves.
I'm not familiar enough with ELF or COFF to know if they should have any relocations marked hidden.

llvm-svn: 142961