Add a comment.

Naively it seemed at first like getVA had the responsibility of adding
the addend, and getSymVA had the responsibility of getting the symbol
VA.
So it was not obvious to me at first why getVA passes Addend to
getSymVA. In fact, it passes it as a mutable reference.

It turns out that it only matters for SHF_MERGE sections, and in
particular only for STT_SECTION symbols that are used as a hack for
reducing the number of local symbols (e.g. to avoid a local symbol for
each string in the string table).

llvm-svn: 296448