Cache the bss output section in the writer, not in the symbol table.
Cache the bss output section in the writer, not in the symbol table.

There is soon going to be two symbol tables, but there will still be only one
output bss.

git-svn-id: f6089bf0e6284f307027cef4f64114ee9ebb0424@247363 91177308-0d34-0410-b5e6-96231b3b80d8

llvm-mirror/lld,llvm-mirror/lld