CodeGen: Don't form weak dllexport aliases

The MS linker cannot do anything interesting with these, it doesn't make
sense to emit them.

This fixes PR21373.

Differential Revision: http://reviews.llvm.org/D5986

git-svn-id: http://llvm.org/svn/llvm-project/cfe/trunk@220595 91177308-0d34-0410-b5e6-96231b3b80d8