fix #21456: foreach over associative array allows incompatible types for key and value

[rainers]

fix the lambda parameter type to correspond to key or value

[dlang-bot]

Thanks for your pull request, @rainers!

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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#21469"

[kinke]

There's a similar case for autodecoding too:

void main() {
    import core.stdc.stdio;
    foreach (int i, dchar c; "abc")
        printf("%d: %c\n", i, c);
}

druntime's _aApplycd2() helper passes the pointers to (original size_t) index and value to the generated foreach-function. And that function then loads a 32-bit integer from the passed index pointer - so truncating to 32-bits on little-endian, but using the 32 upper bits for 64-bit big-endian targets. We have an LDC-specific fix for this (introduced in ldc-developers/ldc@bbdf473); we might be able to fuse these 2 fixes to a single one.

[kinke]

=> #21705