MS ABI: Emit dynamic initializers for selectany globals in comdat-associative sections (PR20889)

This prevents initializers for comdat-folded globals from running multiple times.

Differential Revision: http://reviews.llvm.org/D5281

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@217534 91177308-0d34-0410-b5e6-96231b3b80d8