[mlir][python] Cache import of ir module in type casters. (llvm#160000)

In a JAX benchmark that traces a large language model, this change
reduces the time spent in nanobind::module::import_ from 1.2s to 10ms.

LLVM Release 21.1.2

Release of PRIF lowering for LLVM-HPC25

LLVM Release 21.1.1

LLVM Release 21.1.0

LLVM Release 21.1.0-rc3

[flang][PRIF] Fix default ucobound default value

[flang][PRIF] Fix default ucobound default value

Tagging PRIF branch used for initial paper data collection