[LLDB][ObjectFileELF] Correct the return type of Reloc{Offset,Addend}32

This is a follow up of D145550.

I think Reloc{Type,Symbol}{32,64} can keep unchanged as they are not
directly returning a field of the ELFRel[a] struct.

Reviewed By: DavidSpickett

Differential Revision: https://reviews.llvm.org/D145571

(cherry picked from commit 27705f45)