That commit added a new library just to hold the RawCommentList. I've
started a discussion on the commit thread about whether that is really
meritted -- it certainly doesn't seem necessary at this stage.

However, the immediate problem is that the AST library has a hard
dependency on the Comment library, but the dependencies were set up
completely backward. In addition to the layering violation, this had an
unfortunate effect if scattering the Comments library dependency
throughout the build system, but inconsistently so -- several parts of
the CMake dependencies were missing and only showed up due to transitive
deps or the fact that the target wasn't being built by tho bots.

It turns out that the Comments library can't (currently) be a well
formed layer *below* the AST library either, as it has an API that
accepts an ASTContext. That parameter is currently unused, so maybe that
was a mistake?

Anyways, it really seems like this is logically part of the AST --
that's the whole point of the ASTContext providing access to it as far
as I can tell -- so I've merged it into the AST library to solve the
immediate layering violation problems and remove some of the churn from
our library dependencies.

llvm-svn: 158807