[PECOFF] Handle PRIVATE keyword in the module definition file

A symbol in a module definition file may be annotated with the
PRIVATE keyword like this.

  EXPORTS
    func PRIVATE

The PRIVATE keyword does not affect the resulting .dll file.
But it prevents the symbol to be listed in the .lib (import
library) file.

git-svn-id: https://llvm.org/svn/llvm-project/lld/trunk@218273 91177308-0d34-0410-b5e6-96231b3b80d8