fix issue 19442 - multiple argument string mixin dont support char literals

[ghost]

To support a character, an array literal containing this character is created, which then support the .toStringExp method used later.

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Bugzilla references

Auto-close	Bugzilla	Severity	Description
✓	19442	minor	multiple argument string mixin dont support char literals

Testing this PR locally

If you don't have a local development environment setup, you can use Digger to test this PR:

dub fetch digger
dub run digger -- build "master + dmd#9491"

[thewilsonator]

Love the idea, but D source code is defined to be UTF-8, do we even support wstring and dstring mixins?

[ghost]

Yes, that works, it looks like it's réencoded, for example this is okay

enum xé1 = 42;
enum x2 = mixin('x', wchar('é'), 1);  

[thewilsonator]

Huh. I'll put a 72h-> merge on this in case anyone else wants to chime in.

[ghost]

Addressed the remaining suggestion.
Ping for auto-merge @thewilsonator.