[COFF] Replace OutputSection* with uint16_t index in Chunk

Shaves another 8 bytes off of SectionChunk, the most commonly allocated
type in LLD.

These indices are only valid after we've assigned chunks to output
sections and removed empty sections, so do that in a new pass.

Reviewers: ruiu, aganea

Differential Revision: https://reviews.llvm.org/D62356

llvm-svn: 361657