[mlir][LLVM] Support `immargs` in LLVM_IntrOpBase intrinsics (#73013)

This extends `LLVM_IntrOpBase` so that it can be passed a list of
`immArgPositions` and a list (of the same length) of `immArgAttrNames`.
`immArgPositions` contains the positions of `immargs` on the LLVM IR
intrinsic, and `immArgAttrNames` maps those to a corresponding MLIR
attribute.

This allows modeling LLVM `immargs` as MLIR attributes, which is the
closest match semantically (and had already been done manually for the
LLVM dialect intrinsics).

This has two upsides:
* It's slightly easier to implement intrinsics with immargs now
(especially if they make use of other features, such as overloads)
* It clearly defines that `immargs` should map to attributes, before
there was no mention of `immargs` in LLVMOpBase.td, so implementing them
was unclear

This works with other features of the `LLVM_IntrOpBase`, so `immargs`
can be marked as overloaded too (which is used in some intrinsics).

As part of this patch (and to...