[Issue 12615] Warn against, and then deprecate old alias syntax

[via Digitalmars-d-bugs]

https://issues.dlang.org/show_bug.cgi?id=12615

--- Comment #10 from bearophile_hugs at eml.cc ---
(In reply to Sobirari Muhomori from comment #9)
> Probably leave multialias declaration valid, it's quite handy:
> alias HANDLE BCRYPT_HANDLE, BCRYPT_ALG_HANDLE, BCRYPT_KEY_HANDLE,
> BCRYPT_HASH_HANDLE, BCRYPT_SECRET_HANDLE;

I didn't know this syntax is valid. I think having to define multiple aliases
is not a common need. But if we keep the multialias declaration syntax, then I
think we should keep the single alias syntax too.

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