lld-link: Spelling fixes in comments and minor style tweaks

Changes a few things I noticed while reading this code.
- fix a few typos in comments
- remove two `auto` uses where the type wasn't clear to me
- add comment saying that two sequential checks for `if (SparseChunks[SectionNumber] == PendingComdat)` are intentional
- name two parameters

No behavior change.

Differential Revision: https://reviews.llvm.org/D56677

llvm-svn: 351101