6350: Make IncorrectDiagnostic match rustc by copying rustc's code. r=popzxc a=ArifRoktim
This closes#6343 and closes#6345.
The old algorithm which used a `DetectedCase` enum, didn't match how rustc thinks of cases. Some inputs can be interpreted as more than 1 case depending on the situation. For example, to rustc:
- `ABCD`: Can be both camel case and upper snake case
- `X86_64`: Can be both camel case and upper snake case
I could've made `detect_case` return a collection of `DetectedCase` and then modified the other code as such, but I think using the same code rustc uses is simpler and a surefire way to achieve the same diagnostics as rustc.
Co-authored-by: Arif Roktim <arifrroktim@gmail.com>
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