[PECOFF] Writer: Refactor the chunk class hierarchy.

Before this patch, we had the following class hierarchy.

  Chunk -> AtomChunk -> SectionChunk -> GenericSectionChunk
                                     -> BaseRelocChunk
        -> HeaderChunk

Chunk represented the generic concept of contiguous range in an output
file. AtomChunk represented a chunk consists of atoms.

That class hierarchy had many issues: 1) BaseRelocChunk does not really
consist of atoms, so inheriting from AtomChunk was plainly wrong, and 2)
the hierarchy is unecessarily too deep.

This patch correct them. The new hierachy is shown below.

  Chunk -> SectionChunk -> AtomChunk
                        -> BaseRelocChunk
        -> HeaderChunk

In the new hierarchy, AtomChunk represents a chunk consists of atoms. Other
types of sections (currently only BaseRelocChunk) should inherit directly
from SectionChunk.

git-svn-id: https://llvm.org/svn/llvm-project/lld/trunk@197038 91177308-0d34-0410-b5e6-96231b3b80d8