[PATCH] Bitcode: Add bitcode format compatibility test

[Vedant Kumar]

> LangRef might work, but it would probably be lost among all the stuff there; the argument could also be made that LangRef is basically about the IR itself, but this content is more about development process/tasks.

I buy that argument.

> http://llvm.org/docs/DeveloperPolicy.html#ir-backwards-compatibility is another possibility, which I'm leaning towards, since this is all in connection with compatibility.
> 
> Another possibility is to add it as an explicit task in http://llvm.org/docs/HowToReleaseLLVM.html, but that may implicitly put the burden of doing this on the release manager, which may not be intended (maybe could be added as "track down somebody to do this once we've branched"?).

HowToReleaseLLVM seems a bit high-level for this sort of task. I think the DeveloperPolicy document is a good place.

Here's the wording I picked:

> * The bitcode format produced by a X.Y release will be readable by all
>   following X.Z releases and the (X+1).0 release. To help ensure this, an X.Y
>   version of ``test/Bitcode/compatibility.ll`` should be assembled and
>   committed before each release.


I updated the file paths and comments per Duncan's suggestion. Updated patch attached.

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