COFF: Non-external COMDAT sections sholud not be merged by ICF.

If a section symbol is not external, that COMDAT section should never
be merge with other sections in other compilation unit. Previously,
we didn't take visibility into account.

Note that COMDAT sections with non-external visibility makes sense
because they can be removed by dead-stripping.

Fixes https://llvm.org/bugs/show_bug.cgi?id=25686

llvm-svn: 254578