[lld][ELF] Cleanup ELF writing, No change in functionality

The cleanup includes :-

* Rename ambiguous Header class to ELFHeader
* Convert Chunk contentype and kind to be a enumerated class
* Remove functions that are not being used, avoids future confusion

git-svn-id: https://llvm.org/svn/llvm-project/lld/trunk@189209 91177308-0d34-0410-b5e6-96231b3b80d8