Fix Issue 7352 - Poor error message when using variable as template type parameter

[wolframw]

This change adds supplemental error messages in case a template instance cannot be matched with a declaration. These additional messages are only printed if

• no template constraint evaluation error is printed
• the mismatching argument is either a variable or a function (but not a function literal)

In all other cases the normal error message should be informative enough.

Also, ddoc is added to TemplateInstance.findBestMatch.

[dlang-bot]

Thanks for your pull request and interest in making D better, @wolframw! We are looking forward to reviewing it, and you should be hearing from a maintainer soon.
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• I have provided a detailed rationale explaining my changes
• New or modified functions have Ddoc comments (with Params: and Returns:)

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Bugzilla references

Auto-close	Bugzilla	Severity	Description
✗	7352	normal	Poor error message when using variable as template type parameter

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 + dmd#12289"