Fix Issue 13732 - non-member templates can use "template this"

[ntrel]

Error if a TemplateThisParameter is declared where typeof(this) would be an error.
Error if a TemplateThisParameter is passed a type that doesn't implicitly convert to const typeof(this).

Note: This is done with a dummy specialization, which triggers the semantic error messages even for a template mixin and may be more efficient than doing the check in dtemplate.d.

Explicitly overriding the specialization is supported for now, but even then this T should implicitly convert to const typeof(this).

[dlang-bot]

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Bugzilla references

Auto-close	Bugzilla	Severity	Description
✓	13732	normal	Regular templates can use "template this", and they allow any type to be passed

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#14447"

[ntrel]

This doesn't work because of shared :-/