[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