bpf: btf: Break up btf_type_is_void()

This patch breaks up btf_type_is_void() into
btf_type_is_void() and btf_type_is_fwd().

It also adds btf_type_nosize() to better describe it is
testing a type has nosize info.

Signed-off-by: Martin KaFai Lau <kafai@fb.com>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>