[PATCH] Make .bc en/decoding of AttrKind stable

[Tobias Grosser]

Hi,

I would like to commit the following patch:

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Make .bc en/decoding of AttrKind stable

The bitcode representation attribute kinds are encoded into / decoded 
from should be independent of the current set of LLVM attributes and 
their position in the AttrKind enum. This patch explicitly encodes 
attributes to fixed bitcode values.

With this patch applied, LLVM does not silently misread attributes 
written by LLVM 3.3. We also enhance the decoding slightly such that an 
error message is printed if an unknown AttrKind encoding was dected.

Bonus: Dropping bitcode attributes from AttrKind is now easy, as old 
AttrKinds do not need to be kept to support the Bitcode reader.
----------------------

I also considered assigning fixed numbers directly in the AttrKind enum 
(or keeping the enum stable), but convinced myself that the bitcode
encoding should really not part of the IR interface.

OK to commit?

Cheers,
Tobias
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