[llvm-objdump] - Cleanup the code. NFCI.

This is a cosmetic cleanup for the llvm-objdump code.

This patch:
* Renames things to match the official LLVM code style (lower case -> upper case).
* Removes few obviously excessive variables.
* Moves a few lines closer to the place of use, reorders the code a bit to simplify it,
to avoid doing excessive returns and to avoid using 'else` after returns.

I focused only on a llvm-objdump.h/llvm-objdump.cpp files. Few changes in the
MachODump.cpp and COFFDump.cpp are a result of llvm-objdump.h modification.

Differential revision: https://reviews.llvm.org/D56637

llvm-svn: 351171