Rework _d_arrayappendT to be a template function

[teodutu]

The previous hook was wrapped in a template to easily define _d_arrayappendTTrace as an alias to _d_HookTraceImpl. That implementation forced _d_arrayappendT to be declared @trusted pure to make it callable from pure contexts. This is problematic when the hook calls copyEmplace on a type whose postblit is impure. In this case, copyEmplace is impure, which creates a conflict with _d_arrayappendT's forced purity.

This PR removes the aforementioned template and leaves it up to the compiler to infer the purity of _d_arrayappendT. This change is required for the new lowering of CatAssignExp: dlang/dmd#13495.

[dlang-bot]

Thanks for your pull request and interest in making D better, @teodutu! We are looking forward to reviewing it, and you should be hearing from a maintainer soon.
Please verify that your PR follows this checklist:

• My PR is fully covered with tests (you can see the coverage diff by visiting the details link of the codecov check)
• My PR is as minimal as possible (smaller, focused PRs are easier to review than big ones)
• I have provided a detailed rationale explaining my changes
• New or modified functions have Ddoc comments (with Params: and Returns:)

Please see CONTRIBUTING.md for more information.

---

If you have addressed all reviews or aren't sure how to proceed, don't hesitate to ping us with a simple comment.

Bugzilla references

Your PR doesn't reference any Bugzilla issue.

If your PR contains non-trivial changes, please reference a Bugzilla issue or create a manual changelog.

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#3805"