Report correct location for undefined identifiers in function return types

[abulgit]

Fix: #21165
I use the location stored in the type node (mt.loc) rather than the function declaration location when reporting undefined identifier errors. This ensures that error messages point directly to where the invalid type is declared.
Added test fix21165.d, which verifies the corrected error location. Also Fix an Existing test fail_compilation/noreturn2.d

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[thewilsonator]

Need to update fail_compilation/b23686.d too with

 fail_compilation/b23686.d(107): Error: undefined identifier `eFN1`, did you mean template `eFN0()()`?
 fail_compilation/b23686.d(107): Error: `mixin(_error_)` does not give a valid type
 fail_compilation/b23686.d(115):        while looking for match for `eload!(int, 1)`
-fail_compilation/b23686.d(121): Error: undefined identifier `FNwtf`
+fail_compilation/b23686.d-mixin-121(121): Error: undefined identifier `FNwtf`
 fail_compilation/b23686.d(121): Error: `mixin(_error_)` does not give a valid type
 fail_compilation/b23686.d(126):        while looking for match for `load!"wtf"`

[abulgit]

Yes, got it, but why did the test file name itself change? What is this mixin? This test file looks a bit different from other regular fail_compilation tests. Can you please tell me why this test behaves like this?

[thewilsonator]

No idea, but it is not completely surprising, you are changing the location of the error and the source code for mixins are formatted like that.
The mixin in question is https://github.com/dlang/dmd/blob/master/compiler/test/fail_compilation/b23686.d#L36

Can you please tell me why this test behaves like this?

Why this is affected by a change in the return type? I have no idea. Unless this also affects change in the location of parameters?