[Issue 21074] const lost in mixin

[d-bugmail at puremagic.com]

https://issues.dlang.org/show_bug.cgi?id=21074

moonlightsentinel at disroot.org changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
                 CC|                            |moonlightsentinel at disroot.o
                   |                            |rg
           Severity|normal                      |regression

--- Comment #1 from moonlightsentinel at disroot.org ---
This is a regression from 2.089.1 introduced by this PR:
https://github.com/dlang/dmd/pull/10629

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