[PECOFF] Simplify COFFBSSAtom.

A instance of the class always represents a BSS atom, so we don't need
to look at the symbol or the section to retrieve its attributes.

git-svn-id: https://llvm.org/svn/llvm-project/lld/trunk@187643 91177308-0d34-0410-b5e6-96231b3b80d8
[PECOFF] A symbol with symbol with section number 0 and non-zero value represents a BSS atom.

git-svn-id: https://llvm.org/svn/llvm-project/lld/trunk@187645 91177308-0d34-0410-b5e6-96231b3b80d8