r241653 - Revert "Revert r241620 and follow-up commits" and move the initialization

[Adrian Prantl]

> On Jul 8, 2015, at 9:03 AM, Manuel Klimek <klimek at google.com> wrote:
> 
> On Wed, Jul 8, 2015 at 5:53 PM Adrian Prantl <aprantl at apple.com <mailto:aprantl at apple.com>> wrote:
> > On Jul 8, 2015, at 5:04 AM, Manuel Klimek <klimek at google.com <mailto:klimek at google.com>> wrote:
> >
> > Daniel pointed out this introduces a new dependency onto codegen from tools that only need to parse - was this somehow already there earlier? What does this buy us? (I'm probably missing something :)
> >
> 
> For module debugging we want to emit debug info for the data types defined by a clang module alongside the serialized clang ast when building pch/pcm files. This way we can avoid emitting tons of redundant types in the debug info of each object that was built against the module.
> 
> A tool that wants to parse *and* make use of clang modules or precompiled headers produced by clang will need to link against codegen. Tools that don’t want/need clang modules, or can use a separate module cache can continue to use the RawPCHContainerOperations without introducing any extra dependency. This currently true for all of clang-tools-extra, for example.
> 
> Thanks for explaining, that makes sense. Does debug info really need the full codegen library or only parts of it? (I have no idea how that part works ;)

I could be possible to split out CGDebugInfo, ObjectFilePCHContainerOperations, and BackendUtil into a separate library but looking at the include list in CGDebugInfo it looks like that would also pull in CGBlocks, CGCXXABI, CGObjCRuntime, CodeGenFunction, and CodeGenModule and I think then there is not that much left.
It’s not impossible to split off a data-types-only CGDebugInfo base class to get rid of most of these dependencies, but that’s a larger project. 

-- adrian
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