[PATCH] Avoid crash reading bitcode files with bad abbrev record (bug 20485)

[Rafael Espíndola]

> I chose to put the diagnostic here because that was more consistent with prior use of the API and in particular there are other clients in  llvm/tools/clang (with a separate revision) that have different diagnostic needs.

The code was just changed to even have support for diagnostics. Please
move it to where it is found. With that you can give an error like

"Value XX is not a valid encoding"

Cheers,
Rafael