[cleanup] Random assortment of simple cleanups.
Pruning header includes and forward references and fixing comments.


git-svn-id: https://llvm.org/svn/llvm-project/lld/trunk@197720 91177308-0d34-0410-b5e6-96231b3b80d8
Remove redundant 'classof' functions.


git-svn-id: https://llvm.org/svn/llvm-project/lld/trunk@197726 91177308-0d34-0410-b5e6-96231b3b80d8