add C utility for debugging symbol tables, under #if 0 #109390

In flowgraph.c we have some C functions for dumping instructions and basicblocks as text. These are under #if 0 by default, so effectively comments, but they can be very useful to uncomment and use when debugging a tricky compiler issue.

I recently had to debug a symbol table problem, and I wrote a similar function to (recursively) dump a symbol table and its children to a text representation. I propose that we also include this in the source tree, under #if 0 just like the compiler equivalent, to save time for future debuggers.

The Python symtable module does not fill this need, since you can't use it if the symbol table bug is preventing compilation from even completing.

cc @JelleZijlstra

Linked PRs

• gh-109390: add dump_symtable utility under #if 0 #109391