[clang-format] Skip preprocessor lines when finding the record lbrace

With D117142, we would now format

```
struct A {
#define A
  void f() { a(); }
#endif
};
```

into

```
struct A {
#ifdef A
  void f() {
    a();
  }
#endif
};
```

because we were looking for the record lbrace without skipping preprocess lines.

Fixes https://github.com/llvm/llvm-project/issues/54901.

Reviewed By: curdeius, owenpan

Differential Revision: https://reviews.llvm.org/D123737