[patch][pr17639] Consider alias as definitions

[Rafael Espíndola]

Hi Alp,

Attached is a new version of the patch for pr17639. The differences on
top of the patch you wrote are

* Trying to fix just pr17639 for now to make the patch smaller.
* Handle variables too.
* Remove the warn_redefinition_alias, not clear why it is needed.
* Simpler definition of isDefined. The decl with the alias attribute
is the definition.
* Uses the existing redefinition error when it makes sense.

Cheers,
Rafael
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