[llvm-objdump][COFF][NFC] Split format-specific interfaces; add namespace

Summary:
This patch addresses, for the interfaces implemented by `COFFDump.cpp`,
multiple issues identified with the current structure of
`llvm-objdump.h` in the review of D72973.

This patch moves implementation details of the tool into an
`llvm::objdump` namespace for external linkage names, splits the
implementation details into separate headers for each implementation
file, and uses qualified names when declaring members of the
`llvm::objdump` namespace in place of leaving the namespace definition
open.

Reviewers: jhenderson, DiggerLin, jasonliu, daltenty, MaskRay

Reviewed By: jhenderson, MaskRay

Subscribers: MaskRay, rupprecht, llvm-commits

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D77285