[PECOFF] Recognize but ignore /implib and /safeseh for now.

So that the LLD won't print error message saying that it couldn't find
/implib or /safeseh files.

git-svn-id: https://llvm.org/svn/llvm-project/lld/trunk@195276 91177308-0d34-0410-b5e6-96231b3b80d8