[llvm-objdump] Use `auto` declaration in typecasting

Summary:
According to `MaskRay`, use `auto` for type inference, according to coding standards.
Delete some comments, because these comments can be easily inferred from codes.

Reviewers: jhenderson, MaskRay

Reviewed By: jhenderson

Subscribers: llvm-commits

Differential Revision: https://reviews.llvm.org/D54573

llvm-svn: 346946