Method cache: fix refinement entry handling

	To invalidate some callable method entries, we replace the entry in the
	class. Most types of method entries are on the method table of the
	origin class, but refinement entries without an orig_me are housed in
	the method table of the class itself. They are there because refinements
	take priority over prepended methods.

	By unconditionally inserting a copy of the refinement entry into the
	origin class, clearing the method cache created situations where there
	are refinement entry duplicates in the lookup chain, leading to infinite
	loops and other problems.

	Update the replacement logic to use the right class that houses the
	method entry. Also, be more selective about cache invalidation when
	moving refinement entries for prepend. This avoids calling
	clear_method_cache_by_id_in_class() before refinement entries are in the
	place it expects.

	[Bug #17806]
	---
	 class.c                      |  4 +++-
	 test/ruby/test_refinement.rb | 49 ++++++++++++++++++++++++++++++++++++++++++++
	 vm_method.c                  | 13 ++++++++++--
	 3 files changed, 63 insertions(+), 3 deletions(-)