Fix Issue 20757 - checkaction=context prints characters as integers

[MoonlightSentinel]

No description provided.

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Bugzilla references

Auto-close	Bugzilla	Severity	Description
✓	20757	minor	checkaction=context prints characters as integers

Testing this PR locally

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

dub run digger -- build "master + druntime#3057"

[schveiguy]

It should check for invalid single code units, and print those as "cast(char)255" e.g. Otherwise, the string is going to look weird when printed (and might cause a UTF exception, I'm not sure).

[MoonlightSentinel]

Done