[PECOFF] Change data directory atom structure.

The data directory in the PE/COFF header consisted of list of data directory
atoms. This patch changes it -- now there's only one data directory entry that
contains former data directories. That's easier to handle in the writer as well
as to write to/read from YAML/Native files. The main purpose of this refactoring
is to enable RoundTrip tests for PE/COFF.

There's no functionality change.

llvm-svn: 193854