[PECOFF] Do not write duplicate directives to .def file.

This is a follow-up patch for r220333. r220333 renames exported symbols.
That raised another issue; if we have both decorated and undecorated names
for the same symbol, we'll end up have two duplicate exported symbol
entries.

This is a fix for that issue by removing duplciate entries.

git-svn-id: https://llvm.org/svn/llvm-project/lld/trunk@220350 91177308-0d34-0410-b5e6-96231b3b80d8