[clang] [OpenMP] Add diagnostic for 'factor' width mismatch in 'unroll partial' (PR #139986)

[ALBIN BABU VARGHESE via cfe-commits]

albus-droid wrote:

Thanks for the review. I’ve implemented all the requested changes but still have a few questions:

* Originally, using `IVTy` to query the iteration variable’s width always returned “32-bit int,” even if the loop was declared as `char` or `short`. As a result, errors only appeared when the factor exceeded 32 bits, and the diagnostic always said “int (32 bits).” However, when the loop variable was `long`, the code compiled successfully, so larger types were recognized. I’m not sure whether this was due to how I was reading the type or something else.

* In the current implementation I switched to using `OrigVar->getType()` to get the variable’s `QualType`. Now `getTypeSize()` correctly returns 8 bits for `char`, 16 for `short`, 64 for `long`, etc. That means code like

  ```cpp
  #pragma omp unroll partial(0xFFFFF)
  for (char i = 0; …) { }
  ```

  now rejects, whereas it used to compile.

* One remaining concern: the bit-width check treats all iteration variables as unsigned. It doesn’t account for signed types. Should I add a separate signed-range check, as suggested here in this [comment](https://github.com/llvm/llvm-project/issues/139268#issuecomment-2867068205)?

https://github.com/llvm/llvm-project/pull/139986