[SelectionDAG] Add an ISD::CLEAR_CACHE node to lower llvm.clear_cache (#93795)

The current way of lowering `llvm.clear_cache` is a bit unusual. As
suggested by Matt Arsenault we are better off using an ISD node.

This change introduces a new `ISD::CLEAR_CACHE`, registers a new libcall
by default named `__clear_cache` and the default legalisation is a
libcall.

This is preparatory work for a custom lowering of `ISD::CLEAR_CACHE`
needed by RISC-V on some platforms.