[MC] Add a separate flag for skipping comdat constant sections for MinGW. NFC.

This actually has nothing to do with the associative comdat sections
that aren't supported by GNU binutils ld.

Clarify the comments from SVN r335918 and use a separate flag for it.

Differential Revision: https://reviews.llvm.org/D49645

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@337757 91177308-0d34-0410-b5e6-96231b3b80d8
3 files changed