Make SimpleFileNode inherit from FileNode.

This removes a lot of duplicated code.


git-svn-id: https://llvm.org/svn/llvm-project/lld/trunk@197751 91177308-0d34-0410-b5e6-96231b3b80d8
Fix some typos of 'extension'.


git-svn-id: https://llvm.org/svn/llvm-project/lld/trunk@197753 91177308-0d34-0410-b5e6-96231b3b80d8