[PECOFF] Make PECOFFFileNode::parse idempotent.

PECOFFFileNode::parse can be called twice -- once by WinLink driver and
once more by Driver. We want to make sure that the second call won't mess
up the internal data.

git-svn-id: https://llvm.org/svn/llvm-project/lld/trunk@205284 91177308-0d34-0410-b5e6-96231b3b80d8