fix Bugzilla 24745 - improve associative array syntax error message

[royalpinto007]

Enhance error messages for associative array syntax.

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Bugzilla references

Auto-close	Bugzilla	Severity	Description
✓	24745	enhancement	Better error message when creating an associative array with the wrong syntax

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#16832"

[ntrel]

1. I think the new message should only be shown when detecting a colon after a non-identifier expression. E.g. it looks like the message would show for {expr expr}, which is unlikely to be intended as an array initializer.
2. The message could say (associative) array, because array literals can have indexes too.

Edit: And (1) is why fail_compilation/fix350a.d and fail_compilation/fix350b.d are failing.

[royalpinto007]

I think this can be merged as well? @thewilsonator

no longer affect not assoc arrays