Rollup merge of #87166 - de-vri-es:show-discriminant-before-overflow, r=jackh726
Show discriminant before overflow in diagnostic for duplicate values.
This PR adds the value before overflow for explicit discriminant values in the error for duplicate discriminant values.
I found it rather confusing to see only the overflowed value.
It only does this for literals, since overflows in const evaluated arithmetic are already a hard error.
This is my first PR to the compiler, so please let me know if the implementation can be improved :)
Before:
![image](https://user-images.githubusercontent.com/786213/
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After:
![image](https://user-images.githubusercontent.com/786213/
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