[Clang] Add diagnostic to flag using "%P" specifier with Objective-C pointers

[jaredgrubb]

Darwin has an extension on the printf format-specifiers for "%P", which dumps the bytes at a pointer (in contrast to "%p" which dumps a pointer's value itself) and is allowed in "OS Log" contexts.

It's easy to misuse this, for example:

NSUUID *uuid = ...;
os_log(..., "%{uuid_t}.16P", uuid);

This dumps the ObjC class data (eg, the is-a pointer) -- not the UUID represented by the object.

It'd be nice if the compiler had a warning that could flag this misuse, which seems to always (IMO) be a bug.

[jaredgrubb]

I will put up a PR shortly to propose a new diagnostic for this.