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

[Adrian Prantl]

> On Jul 8, 2015, at 5:04 AM, Manuel Klimek <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.

-- adrian