common: fix TrackedOp intrusive_ptr compatibility with boost 1.89+

common: fix TrackedOp intrusive_ptr compatibility with boost 1.89+

Boost 1.89+ includes a new header sp_cxx20_constexpr.hpp (Copyright 2025)
that defines BOOST_SP_CXX20_CONSTEXPR macro. When building with C++20/23
mode and a compiler supporting constexpr dynamic allocation, this macro
expands to 'constexpr', making intrusive_ptr constructors and destructors
constexpr.

This change breaks builds that previously worked with boost 1.87 because:

In boost 1.87, the copy constructor was NOT constexpr:
  intrusive_ptr(intrusive_ptr const & rhs): px( rhs.px )
  {
      if( px != 0 ) intrusive_ptr_add_ref( px );
  }

In boost 1.89+ with C++20/23, it becomes constexpr:
  BOOST_SP_CXX20_CONSTEXPR intrusive_ptr(intrusive_ptr const & rhs): px( rhs.px )
  {
      if( px != 0 ) intrusive_ptr_add_ref( px );
  }

With constexpr, name lookup for intrusive_ptr_add_ref happens at compile
time during template instantiation, not at runtime. This changes the
lookup behavior significantly.

The issue is the "hidden friend" pattern: friend functions defined inside
a class are in the enclosing (global) namespace, but per C++ standard
[basic.lookup.argdep], they are ONLY visible via ADL (Argument-Dependent
Lookup), not via ordinary unqualified lookup.

When boost::intrusive_ptr's constexpr constructor tries to call
intrusive_ptr_add_ref(px) during template instantiation:
1. Ordinary unqualified lookup finds ceph::common::intrusive_ptr_add_ref
2. Since ordinary lookup succeeded, ADL is not performed [basic.lookup.argdep]/1
3. The friend functions in TrackedOp are never considered
4. Compilation fails due to signature mismatch (TrackedOp* vs RefCountedObject*)

In boost 1.87 (non-constexpr), ADL worked normally at runtime and found
the hidden friend functions. With constexpr in 1.89+, compile-time lookup
finds the wrong function before ADL can trigger.

The fix adds forward declarations before boost::intrusive_ptr<TrackedOp>
is first used. This makes the functions visible to ordinary lookup (not
just ADL), allowing the compiler to find them instead of the ceph::common
versions. The friend functions provide the actual definitions.

Note: Friend functions defined inside a class are already implicitly
inline per the C++ standard, so no explicit inline specifier is needed
on the friend function definitions.

This issue manifests when building with:
- Boost 1.89+ (which introduced sp_cxx20_constexpr.hpp)
- C++23 standard mode
- Compiler with constexpr dynamic allocation support

Fixes build errors like:
  error: 'intrusive_ptr_add_ref' was not declared in this scope

Signed-off-by: Kefu Chai <k.chai@proxmox.com>