Fix issue 20545 - Segfault/Assertion failure when parsing invalid AA literal

[thewilsonator]

No description provided.

[dlang-bot]

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Bugzilla references

Auto-close	Bugzilla	Severity	Description
✓	20545	normal	Segfault/Assertion failure when parsing invalid AA literal

Testing this PR locally

If you don't have a local development environment setup, you can use Digger to test this PR:

dub fetch digger
dub run digger -- build "master + dmd#10749"

[WalterBright]

see refactoring comment