Adding LLVM-21 Support

[michail-boulasikis]

This pull request changes the way Attribute::NoCapture is handled when LLVM MAJOR >= 21. This makes RAPTOR compile using LLVM 21.

[ivanradanov]

Adding helper functions like

addNoCapture(CallInst* CI, unsigned ArgNo);

and similar for Function and putting the #ifdefs inside the functions might be nicer so as not to repeat the same thing everywhere.

Also if you could add llvm 21 to the testing matrix here, we can also make sure to check it in CI

[michail-boulasikis]

Thanks for the good suggestions, I have now implemented them.