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

[d-bugmail at puremagic.com]

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

--- Comment #20 from Mike Franklin <slavo5150 at yahoo.com> ---
The only reason `alias whetever this` does not use the assignment syntax is
that the `alias this = existingname` conflicts with the ability to alias base
(i.e. super class) constructors: `alias this = super.this`.  Find a solution to
that and we can finally use the assignment syntax for alias this.

See discussions at https://github.com/dlang/dlang.org/pull/200 and
https://github.com/dlang/dmd/pull/1341 for greater understanding of the issue.

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