[MLIR] Add index bitwidth to the DataLayout (#85927)

When importing from LLVM IR the data layout of all pointer types
contains an index bitwidth that should be used for index computations.
This revision adds a getter to the DataLayout that provides access to
the already stored bitwidth. The function returns an optional since only
pointer-like types have an index bitwidth. Querying the bitwidth of a
non-pointer type returns std::nullopt.

The new function works for the built-in Index type and, using a type
interface, for the LLVMPointerType.