Use Intrinsic::ID instead of unsigned. NFC.

This is after LLVM r237810 which made Function::getIntrinsicID() return an Intrinsic::ID.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@237811 91177308-0d34-0410-b5e6-96231b3b80d8