[Writer] Rename variables to reflect reality. NFCI.

The InputSection variables in the Writer were named `C`. This
was because when the ELF linker was ported (from COFF)
the name `Chunks` for input sections was retained.
Luckily we switched to a more ELF-compliant jargon, but these
variables weren't reanamed accordingly during the transition.

llvm-svn: 281917