Fix bug#30846, along with misc cleanups found along the way

* test/src/data-tests.el (data-tests-kill-all-local-variables): New test.

* src/buffer.c (swap_out_buffer_local_variables): Remove.
Fuse the body of its loop into that of reset_buffer_local_variables.
(Fkill_buffer, Fkill_all_local_variables): Don't call it any more.
(reset_buffer_local_variables): Make sure the buffer's local binding
is swapped out before removing it from the alist (bug#30846).
Call watchers before actually killing the var.

* src/data.c (Fmake_local_variable): Simplify.
Use swap_in_global_binding to swap out any local binding, instead of
a mix of find_symbol_value followed by messing with where&found.
Don't call swap_in_symval_forwarding since the currently swapped
binding is never one we've modified.
(Fkill_local_variable): Use swap_in_global_binding rather than messing
with where&found to try and trick find_symbol_value into doing the same.

* src/alloc.c (mark_localized_symbol): 'where' can't be a frame any more.

(cherry picked from commit 3ddff080)
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