Fix undo of changes in cloned indirect buffers

* lisp/simple.el (primitive-undo): If the visited-modtime of the
indirect buffer's file is bogus, use the modtime of the file
visited by its base buffer.

* src/undo.c (record_first_change): Call
'buffer_visited_file_modtime' with the correct buffer, instead of
always calling 'Fvisited_file_modtime', which returns possibly
bogus values for indirect buffers.
* src/fileio.c (Fset_visited_file_modtime): Signal a meaningful
error for indirect buffers.
(buffer_visited_file_modtime): New function, with implementation
taken from 'Fvisited_file_modtime'.
(Fvisited_file_modtime): Call 'buffer_visited_file_modtime'.
* src/lisp.h: Add prototype for 'buffer_visited_file_modtime'.
(Bug#56397)

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