[clang] [clang-format]: Treat #pragma once as include guard for IndentPPDirectives (PR #135443)

[Björn Schäpers via cfe-commits]

HazardyKnusperkeks wrote:

But it should be reasonable. Why combine `#pragma once` and an `#ifndef` include guard? That's the worst of both worlds.

https://github.com/llvm/llvm-project/pull/135443