[clang] [clang][Sema] Stop format size estimator upon %p to adapt to linux kernel's extension (PR #65969)

[Takuya Shimizu via cfe-commits]

hazohelet wrote:

Thanks for the detailed suggestion! I agree it's the appropriate approach.

I was planning to separate `Wformat-truncation` from `Wfortify-source` sometime after this PR (as was requested in the same issue), but now that we are going to introduce a new warning group, we should finish the separation before proceeding further on this.
Sadly we don't have any way to make stacked patches as of now, so I'll merge the separation change to this PR. If reviewers think it's unreasonable, let's make another PR for the separation then.

https://github.com/llvm/llvm-project/pull/65969