[PECOFF] Do not print @<ordinal> if the symbol is private

lib.exe prints a warning if a symbol in a module definition file has
both the PRIVATE attribute and an ordinal like this.

  EXPORTS
    foo @1 PRIVATE

This patch suppresses that.

git-svn-id: https://llvm.org/svn/llvm-project/lld/trunk@218395 91177308-0d34-0410-b5e6-96231b3b80d8