Fix #19163 - ICE with delegate in UDA

[vindexbit]

Related issue: #19163

dmd sometimes lowered a delegate literal in a UDA (retrieved via __traits(getAttributes)) as only a function pointer (one word) and then assigned it into a delegate slot (two words).

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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#22316"

[thewilsonator]

otherwise looks good

[thewilsonator]

compilable tests shouldn't have their test function be called main

[vindexbit]

This command shows a suspiciously large number of fails:
HOST_DMD=../../generated/linux/release/64/dmd ./run.d compilable

...
FAILED targets:
- compilable/issue15574.sh
- compilable/test14894.sh
- compilable/issue19243.sh
- compilable/issue19724.sh
- compilable/test6461.sh
- compilable/issue24830.sh