Remove COFFYAML::Header.

Instead, use MappingNormalization to directly parse COFF::header. Also change
the naming convention of the helper classes to be a bit shorter.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@179917 91177308-0d34-0410-b5e6-96231b3b80d8