[sancov] Put .SCOV* sections into the right comdat groups on COFF

Avoids linker errors about relocations against discarded sections.

This was uncovered during the Chromium clang roll here:
https://chromium-review.googlesource.com/c/chromium/src/+/1321863#message-717516acfcf829176f6a2f50980f7a4bdd66469a

After this change, Chromium's libGLESv2 links successfully for me.

Reviewers: metzman, hans, morehouse

Differential Revision: https://reviews.llvm.org/D54232

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@346381 91177308-0d34-0410-b5e6-96231b3b80d8