undefined: Neater CallStack in error message

Users of `undefined` don’t want to see
```
files.hs: Prelude.undefined:
CallStack (from HasCallStack):
  error, called at libraries/base/GHC/Err.hs:79:14 in base:GHC.Err
  undefined, called at file.hs:151:19 in main:Main
```
but want to see
```
files.hs: Prelude.undefined:
CallStack (from HasCallStack):
  undefined, called at file.hs:151:19 in main:Main
```
so let’s make that so.

The function for that is `withFrozenCallStack`, but that is not usable
here (module dependencies, and also not representation-polymorphic). And
even if it were, it could confuse GHC’s strictness analyzer, leading to
big regressions in some perf tests (T10421 in particular).

So after shuffling modules and definitions around, I eventually noticed
that the easiest way is to just not call `error` here.

Fixes #19886

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