Recommit r324455 "[ThinLTO] - Simplify code in ThinLTOBitcodeWriter."

With fix: reimplemented.

Original commit message:
Recently introduced convertToDeclaration is very similar
to code used in filterModule function.
Patch reuses it to reduce duplication.

Differential revision: https://reviews.llvm.org/D42971

llvm-svn: 324574