IWYU pragmas can be misleading in presence of forward declarations

[kadircet]

foo_priv.h:

// IWYU pragma: private, include "foo.h"
class Bar;
class Foo {};

foo.h:

#include "foo_priv.h"

bar_header.h:

class Bar {};

Despite of foo.h not owning Bar, we'll infer it as canonical header for Bar, because of the IWYU private pragma.

I guess we can adjust private pragma to only affect declarations with a definition.

The other alternative is changing the strength for canonical signal, but it might regress other cases.

[llvmbot]

@llvm/issue-subscribers-clang-include-cleaner

[kadircet]

https://reviews.llvm.org/D159441