ThinLTOBitcodeWriter: keep comdats together, rename if leader is renamed

Summary:
COFF requires that every comdat contain a symbol with the same name as
the comdat. ThinLTOBitcodeWriter renames symbols, which may cause this
requirement to be violated. This change avoids such violations by
renaming comdats if their leaders are renamed. It also keeps comdats
together when splitting modules.

Reviewers: pcc, mehdi_amini, tejohnson

Reviewed By: pcc

Subscribers: rnk, Prazek, llvm-commits

Differential Revision: https://reviews.llvm.org/D31963

llvm-svn: 300019