libbpf: Resolve unambigous forward declarations

Resolve forward declarations that don't take part in type graphs
comparisons if declaration name is unambiguous. Example:

CU #1:

struct foo;              // standalone forward declaration
struct foo *some_global;

CU #2:

struct foo { int x; };
struct foo *another_global;

The `struct foo` from CU #1 is not a part of any definition that is
compared against another definition while `btf_dedup_struct_types`
processes structural types. The the BTF after `btf_dedup_struct_types`
the BTF looks as follows:

[1] STRUCT 'foo' size=4 vlen=1 ...
[2] INT 'int' size=4 ...
[3] PTR '(anon)' type_id=1
[4] FWD 'foo' fwd_kind=struct
[5] PTR '(anon)' type_id=4

This commit adds a new pass `btf_dedup_resolve_fwds`, that maps such
forward declarations to structs or unions with identical name in case
if the name is not ambiguous.

The pass is positioned before `btf_dedup_ref_types` so that types
[3] and [5] could be merged as a same type after [1] and [4] are merged.
The final result for the example above looks as follows:

[1] STRUCT 'foo' size=4 vlen=1
	'x' type_id=2 bits_offset=0
[2] INT 'int' size=4 bits_offset=0 nr_bits=32 encoding=SIGNED
[3] PTR '(anon)' type_id=1

For defconfig kernel with BTF enabled this removes 63 forward
declarations. Examples of removed declarations: `pt_regs`, `in6_addr`.
The running time of `btf__dedup` function is increased by about 3%.

Signed-off-by: Eduard Zingerman <eddyz87@gmail.com>
Signed-off-by: Andrii Nakryiko <andrii@kernel.org>
Reviewed-by: Alan Maguire <alan.maguire@oracle.com>
Acked-by: Andrii Nakryiko <andrii@kernel.org>
Link: https://lore.kernel.org/bpf/20221109142611.879983-3-eddyz87@gmail.com