Simplify yaml2obj a bit.

The COFFParser now contains only a COFFYAML::Object and the string table
(which is recomputed, not serialized).

The structs in COFFParser now all begin with a Header field with what is
actually on the COFF object. The other fields are things that are semantically
part of the struct (relocations in a section for exmaple), but are not actually
represented that way in the object file.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@180134 91177308-0d34-0410-b5e6-96231b3b80d8