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

[Owen Pan via cfe-commits]

owenca wrote:

Why would one want to use `pragma once` above a header guard? In fact, if I have the following,
```
#pragma once

#ifndef MY_MACRO
#define MY_MACRO
#endif

// ...
```
I don't want that to be treated like a header guard.

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