[COFF] Fix constant sharing regression for MinGW

This fixes a regression since SVN r334523, where the object files
built targeting MinGW were rejected by GNU binutils tools. Prior to
that commit, we only put constants in comdat for MSVC configurations.

Differential Revision: https://reviews.llvm.org/D48567

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@335918 91177308-0d34-0410-b5e6-96231b3b80d8
2 files changed