[llvm-readef/obj] - Change the design structure of ELF dumper. NFCI.

This is a refactoring for design of stuff in `ELFDumper.cpp`.
The current design of ELF dumper is far from ideal.

Currently most overridden functions (inherited from `ObjDumper`) in `ELFDumper` just forward to
the functions of `ELFDumperStyle` (which can be either `GNUStyle` or `LLVMStyle`).
A concrete implementation may be in any of `ELFDumper`/`DumperStyle`/`GNUStyle`/`LLVMStyle`.

This patch reorganizes the classes by introducing `GNUStyleELFDumper`/`LLVMStyleELFDumper`
which inherit from `ELFDumper`. The implementations are moved:

`DumperStyle` -> `ELFDumper`
`GNUStyle` -> `GNUStyleELFDumper`
`LLVMStyle` -> `LLVMStyleELFDumper`

With that we can avoid having a lot of redirection calls and helper methods.
The number of code lines changes from 7142 to 6922 (reduced by ~3%) and the
code overall looks cleaner.

Differential revision: https://reviews.llvm.org/D93900