COFF: Devirtualize mark(), markLive() and isCOMDAT().

Only SectionChunk can be dead-stripped. Previously,
all types of chunks implemented these functions,
but their functions were blank.

Likewise, only DefinedRegular and DefinedCOMDAT symbols
can be dead-stripped. markLive() function was implemented
for other symbol types, but they were blank.

I started thinking that the change I made in r240319 was
a mistake. I separated DefinedCOMDAT from DefinedRegular
because I thought that would make the code cleaner, but now
we want to handle them as the same type here. Maybe we
should roll it back.

This change should improve readability a bit as this removes
some dubious uses of reinterpret_cast. Previously, we
assumed that all COMDAT chunks are actually SectionChunks,
which was not very obvious.

llvm-svn: 240675