pruning unused debugging types (enums/PR23336)

[Aldy Hernandez]

> You could combine the two ideas: a global hash table of types used in
> casts, where each entry had a list of functions using those types.  That
> should take up no more storage than the per-function vectors.  Then,
> you'd have to walk the entire hash table, writing out each type for
> which at least one of the associated functions was written out,
> including being inlined into another function.

Do we keep a hash of functions that have been written out somewhere?
I'd hate to walk the entire hash table each time we write out a function
searching for the types that function uses.

Aldy