Fix 19783 - Fail to emplace struct with betterC

[MoonlightSentinel]

Defining the helper function as a template ensures that it is available when not linking druntime.

(pragma(inline, true) didn't work)

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Bugzilla references

Auto-close	Bugzilla	Severity	Description
✓	19783	normal	Fail to emplace struct with betterC

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#3454"

[kinke]

Templatizing it isn't actually enough to guarantee that it's emitted (template culling, needsCodegen()), but yeah, it works in most cases. I'm absolutely no friend of these dummy templates; I don't want to end up with a templates-only druntime/Phobos (STL) just to allow people to use druntime/Phobos stuff with -betterC. People can e.g. still link druntime manually with -betterC - that'll most likely result in some bloat, unused TypeInfos and ModuleInfos etc., but still, missing symbols are resolved, and unless the called functions require a properly initialized druntime (module ctors etc.), it actually works.

[MoonlightSentinel]

Fair enough. I've inlined those asserts because the helper function was only used in two places and this save some unnecessary template instantiations.