[PECOFF][Writer] s/dyn_cast/cast/ where we don't expect null return value.

git-svn-id: https://llvm.org/svn/llvm-project/lld/trunk@186353 91177308-0d34-0410-b5e6-96231b3b80d8
[PECOFF][Writer] Follow up patch for r186336.

- Make a const reference instead of copying an object
- Fix a comment

git-svn-id: https://llvm.org/svn/llvm-project/lld/trunk@186355 91177308-0d34-0410-b5e6-96231b3b80d8