[llvm-dwarfdump] Add the --show-sections-sizes option

Add an option to llvm-dwarfdump to calculate the bytes within
the debug sections. Dump this numbers when using --statistics
option as well.

This is an initial patch (e.g. we should support other units,
since we only support 'bytes' now).

Differential Revision: https://reviews.llvm.org/D74205