[Clang] Add diagnostic about "%P" specifier with Objective-C pointers (#89977)

A Darwin extension '%P' combined with an Objective-C pointer seems to
always be a bug.

'%P' will dump bytes at the pointed-to address (in contrast to '%p'
which dumps the pointer itself). This extension is only allowed in "OS
Log" contexts and is intended to be used like `%{uuid_t}.*16P` or
`%{timeval}.*P`. If an ObjC pointer is used, then the internal runtime
structure (aka, the is-a pointer and other runtime metadata) will be
dumped, which (IMO) is never the expectation.

A simple diagnostic can help flag these scenarios.

Resolves https://github.com/llvm/llvm-project/issues/89968



Co-authored-by: Jared Grubb <jgrubb@apple.com>