[llvm-dwp] Refactor handling of section identifiers. NFCI.

There is a number of places in llvm-dwp.cpp where a section identifier
is translated into an index of an internal array of section
contributions, and another place where the index is converted to an
on-disk value. All these places use direct expressions like
"<id> - DW_SECT_INFO" or "<index> + DW_SECT_INFO", exploiting the fact
that DW_SECT_INFO is the minimum valid value of that kind.

The patch adds distinct functions for that translation. The goal is to
make the code more readable and to prepare it to support index sections
of new versions, where the numeric scheme of section indexes is changed.

Differential Revision: https://reviews.llvm.org/D76067