[patch][pr10367] Add support for offsets in GlobalAlias

[Tim Northover]

Hi Rafael,

Mostly this looks fine, I think. Just one thing missing that I
spotted: updates to the documentation. On your specific questions:

> Particular items it would be nice to have a second opinion on:
>
> * LL Syntax
> * Bitcode representation

Both of these look fine to me. About the only bike-shed around is
probably whether there should be a comma, but personally I think I
agree with you omitting it.

> * Should offsets past the end of an object be illegal or we just don't
> define what the result is?

Hmm. You can llvm-link two modules together with incompatible
definitions of the same global, so whether an alias is out of range
might not be a simple question to answer if that has valid uses.

If not, though, I'd probably choose illegal. All current potential
uses seem to be compiler-internal where we (hopefully) know the offset
is valid.

Cheers.

Tim.