[CodeGen] Clarify comment about COFF common symbol alignment

After a discussion on the commit thread, it seems the 32 byte alignment
limitation is an MSVC toolchain artifact, not an inherent COFF
restriction. Clarify the comment accordingly, since saying COFF in the
comment but using isKnownWindowsMSVCEnvironment in the conditional is
confusing. Also add a newline before the comment, which is consistent
with the local style.

Differential Revision: https://reviews.llvm.org/D56466

llvm-svn: 350754