libbpf: Avoid enum forward-declarations in public API in C++ mode

libbpf: Avoid enum forward-declarations in public API in C++ mode

[ Upstream commit 039f12976fc4a549fa1ef1692f3b742499eb0cbd ]

C++ enum forward declarations are fundamentally not compatible with pure
C enum definitions, and so libbpf's use of `enum bpf_stats_type;`
forward declaration in libbpf/bpf.h public API header is causing C++
compilation issues.

More details can be found in [0], but it comes down to C++ supporting
enum forward declaration only with explicitly specified backing type:

  enum bpf_stats_type: int;

In C (and I believe it's a GCC extension also), such forward declaration
is simply:

  enum bpf_stats_type;

Further, in Linux UAPI this enum is defined in pure C way:

enum bpf_stats_type { BPF_STATS_RUN_TIME = 0; }

And even though in both cases backing type is int, which can be
confirmed by looking at DWARF information, for C++ compiler actual enum
definition and forward declaration are incompatible.

To eliminate this problem, for C++ mode define input argument as int,
which makes enum unnecessary in libbpf public header. This solves the
issue and as demonstrated by next patch doesn't cause any unwanted
compiler warnings, at least with default warnings setting.

  [0] https://stackoverflow.com/questions/42766839/c11-enum-forward-causes-underlying-type-mismatch
  [1] Closes: https://github.com/libbpf/libbpf/issues/249

Signed-off-by: Andrii Nakryiko <andrii@kernel.org>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Link: https://lore.kernel.org/bpf/20221130200013.2997831-1-andrii@kernel.org
Signed-off-by: Sasha Levin <sashal@kernel.org>