[patch] Simplify the datalayout strings by using the defaults

[Rafael Espíndola]

>> By the way, the documentation (at http://llvm.org/docs/LangRef.html#data-layout) doesn't say the default value of <pref> (in i<size>:<abi>:<pref>). It's reasonable to assume <abi> but it ought to say that. Similarly it doesn't say the meaning of <size> (in a<size>:<abi>:<pref> and s<size>:<abi>:<pref>) when it is zero or omitted. Could you add a sentence or two to explain it?
>
> The size is never used. The attached patch updates the documentation
> to always omit the size of 's' and 'a' and adds an assert to check
> that they are always 0.

A rebased patch is attached. What do you think?

Cheers,
Rafael
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