[mlir] Add option to use custom base class for dialect in LLVMIRIntrinsicGen.

Summary:
LLVMIRIntrinsicGen is using LLVM_Op as the base class for intrinsics.
This works for LLVM intrinsics in the LLVM Dialect, but when we are
trying to convert custom intrinsics that originate from a custom
LLVM dialect (like NVVM or ROCDL) these usually have a different
"cppNamespace" that needs to be applied to these dialect.

These dialect specific characteristics (like "cppNamespace")
are typically organized by creating a custom op (like NVVM_Op or
ROCDL_Op) that passes the correct dialect to the LLVM_OpBase class.

It seems natural to allow LLVMIRIntrinsicGen to take that into
consideration when generating the conversion code from one of these
dialect to a set of target specific intrinsics.

Reviewers: rriddle, andydavis1, antiagainst, nicolasvasilache, ftynse

Subscribers: jdoerfert, mehdi_amini, jpienaar, burmako, shauheen, arpith-jacob, mgester, lucyrfox, aartbik, liufengdb, llvm-commits

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D73233