Previously, live range of `ast_value` ended on the call right before
rb_ast_dispose(), which led to premature collection and use-after-free.

We observed this crashing on -O3, -DVM_CHECK_MODE, with GCC 11.4.0 on
Ubuntu.

Co-authored-by: Aaron Patterson <tenderlove@ruby-lang.org>

Previously, vm_make_env_each() (used during proc
creation and for the debug inspector C API) picked up the
non-GC-allocated iseq that rb_vm_push_frame_fname() creates,
which led to a SEGV when the GC tried to mark the non GC object.

Put a real iseq imemo instead. Speed should be about the same since
the old code also did a imemo allocation and a malloc allocation.

Real iseq allows ironing out the special-casing of dummy frames in
rb_execution_context_mark() and rb_execution_context_update(). A check
is added to RubyVM::ISeq#eval, though, to stop attempts to run dummy
iseqs.

[Bug #21180]

Co-authored-by: Aaron Patterson <tenderlove@ruby-lang.org>