ThinLTOBitcodeWriter: handle aliases first in filterModule

Summary: This change fixes a "local linkage requires default visibility" assert when attempting to build LLVM with ThinLTO on Windows.

Reviewers: pcc, tejohnson, mehdi_amini

Reviewed By: pcc

Subscribers: llvm-commits, Prazek

Differential Revision: https://reviews.llvm.org/D31632

llvm-svn: 299491