[clang][XCOFF] Indicate that XCOFF does not support COMDATs

Summary: XCOFF doesn't support COMDATs, so clang shouldn't emit them.

Reviewers: stevewan, sfertile, Xiangling_L

Reviewed By: sfertile

Subscribers: dschuff, aheejin, dexonsmith, cfe-commits, llvm-commits

Tags: #clang, #llvm

Differential Revision: https://reviews.llvm.org/D74631