[LLVM][TableGen] Change CallingConvEmitter to use const RecordKeeper (#108955)
Change CallingConvEmitter to use const RecordKeeper.
This is a part of effort to have better const correctness in TableGen
backends:
https://discourse.llvm.org/t/psa-planned-changes-to-tablegen-getallderiveddefinitions-api-potential-downstream-breakages/81089