Before this commit, we assigned names to struct types without looking at
the function names of which they were return types.
Now we do, so that the name of the generated type for the function `f`
is `f_ret_type`.
This also fixes errors when reparsing the generated C code, where the
names of the struct types did not match.
Replicate the changes made by this commit
f2a0df309f78e1b5d7c5f81ada5110523644559e perfomed by Pietro on the
branch containing the development fixes for `revng-c`.
Pietro Fezzardi