gh-102654: Insert #line directives in generated_cases.c.h (#102669)

This behavior is optional, because in some extreme cases it
may just make debugging harder. The tool defaults it to off,
but it is on in Makefile.pre.in.

Also note that this makes diffs to generated_cases.c.h noisier,
since whenever you insert or delete a line in bytecodes.c,
all subsequent #line directives will change.