btf: do not prune at typedefs

The existing BTF pruning logic meant that an anonymous struct or
union type hidden behind a typedef, such as in the common construct:

  typedef struct { ... } my_struct_type;

could be pruned if 'my_struct_type' was only ever referenced via pointer
members in other structs/unions types used in the program.

The result of pruning is to skip emitting full type information for
a struct or union type by replacing it with a BTF_KIND_FWD, indicating
that it exists but its definition is omitted.  Any types used only by
pruned types are fully omitted from the generated BTF.

In cases like this where the struct/union type is anonymous, the result
is an anonymous BTF_KIND_FWD, which is useless.  The presence of such a
type record rightly causes complaints from BTF loaders.  Worse, since
the TYPEDEF for 'my_struct_type' itself may _not_ be pruned, its type
information will be incomplete.

Change the BTF pruner so that we never consider pruning at a typedef,
and always either keep or discard both the type and the typedef.

gcc/

	* btfout.cc (btf_add_used_type_1): Do not consider creating
	fixups at typedefs.

gcc/testsuite/

	* gcc.dg/debug/btf/btf-prune-4.c: New.

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