Merge all Hydrogen phase declarations into hydrogen-phases.h.

Merge all Hydrogen phase declarations into hydrogen-phases.h.

See https://codereview.chromium.org/18562009 for the discussion.

[Sven Panne, 2013-07-22 07:15:04]

Strong -1 from my side. Although this is a kind of bike shed discussion, I very
strongly favor the 1:1 *.cc/*.h relationship with small headers: Let's keep
separate things separate. Instead of this rather random merge into a single
header, simply put all phases (plus perhaps some other stuff) into a
subdirectory "hydrogen". In general, we should do this far more often, e.g. some
subdir for general data structures etc.

[titzer, 2013-07-22 07:15:04]

lgtm

[danno, 2013-07-22 08:00:01]

I tend to agree with Sven on this one. The more I think about it, the less I
like the single phase header... I see no real advantage and just a big
disadvantage--not being able to quick jump between the header and source file.

I think moving all of the compiler-related files into a hydrogen directory helps
keep a handle on the files-per-directory explosio and I'd prefer doing that
first.