do not lower if using CTFE

[WalterBright]

This lowering causes a lot of template bloat and slowdown and is not necessary for CTFE execution. CTFE ought to be able to execute new directly.

[dlang-bot]

Thanks for your pull request, @WalterBright!

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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#14940"

[WalterBright]

Turns out that CTFE ignores the lowering. However, the templates still get instantiated and since the instantiations use the GC this causes problems with -betterC. This PR turns off those instantiations.

[WalterBright]

ping @RazvanN7