Change Function::getIntrinsicID() to return an Intrinsic::ID. NFC.

Change Function::getIntrinsicID() to return an Intrinsic::ID.  NFC.

Now that Intrinsic::ID is a typed enum, we can forward declare it and so return it from this method.

This updates all users which were either using an unsigned to store it, or had a now unnecessary cast.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@237810 91177308-0d34-0410-b5e6-96231b3b80d8