Fix 20604 - [ICE] dtoh ICE with nested template structs

[MoonlightSentinel]

Define a handler for TypeInstance and handle potentially missing template identifiers.

(Extracted from #11848)

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Currently not targeting stable because the issue was marked as major because of the CI role and all recent improvements were merged to master.

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Bugzilla references

Auto-close	Bugzilla	Severity	Description
✓	20604	major	[ICE] dtoh ICE with nested template structs (and probably most templates

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#11875"