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

[Rafael Espíndola]

This patch completes the implementation of pr10367. With it an alias
can reliably
point to an offset of another object.

This is needed for RTTI on windows where the RTTI data precedes the
vtable symbol. It also seems desirable for concatenating globals used
from a single function to improve ARM and AARCH64 code.

Particular items it would be nice to have a second opinion on:

* LL Syntax
* Bitcode representation
* I included tests for llvm-as/llvm-dis, llvm-link and llc, but let me
know if there is any other test you would find interesting.
* Should offsets past the end of an object be illegal or we just don't
define what the result is?

Cheers,
Rafael
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