Improve the --expand-relocs handling of MachO.

In a relocation target can take 3 basic forms

* A r_value in scattered relocations.
* A symbol in external relocations.
* A section is non-external relocations.

Have the dump reflect that. With this change we go from

CHECK-NEXT:       Extern: 0
CHECK-NEXT:       Type: X86_64_RELOC_SUBTRACTOR (5)
CHECK-NEXT:       Symbol: 0x2
CHECK-NEXT:       Scattered: 0

To just

// CHECK-NEXT:       Type: X86_64_RELOC_SUBTRACTOR (5)
// CHECK-NEXT:       Section: __data (2)

Since the relocation is with a section, we print the seciton name and don't
need to say that it is not scattered or external.

Someone motivated can add further special cases for things like
ARM64_RELOC_ADDEND and ARM_RELOC_PAIR.

llvm-svn: 240073