[Issue 21391] New: deprecated doesn't work for mixin templates

[d-bugmail at puremagic.com]

https://issues.dlang.org/show_bug.cgi?id=21391

          Issue ID: 21391
           Summary: deprecated doesn't work for mixin templates
           Product: D
           Version: D2
          Hardware: All
                OS: All
            Status: NEW
          Severity: normal
          Priority: P1
         Component: dmd
          Assignee: nobody at puremagic.com
          Reporter: mrsmith33 at yandex.ru

Minimal test case (1 deprecation expected):
```
void main(string[] args) {}
mixin Test!("hello", 42);
deprecated mixin template Test(Args...){ int b; }
```

Complex test case. No deprecation messages occur (2 expected)

```
// Check for local and static template mixins
void main(string[] args) {
    mixin Test!("hello", 42);
    mixin Test!(Foo()); // deprecated expected
    mixin Test!(50.0, 42);
    a = b + c + .a + .b + .c;
}
mixin Test!("hello", 42);
mixin Test!(Foo()); // deprecated expected
mixin Test!(50.0, 42);

struct Foo {}

// 3 overloads, so any ordering works
mixin template Test(double num, Args...){ int a; }
deprecated
mixin template Test(Args...){ int b; }
mixin template Test(string str, Args...){ int c; }
```

No deprecation messages are issued by the compiler here.

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