[[clang-format] Main header search algorithm should consider directories, not only file names

[EugeneZelenko]

Test case (test/gui/application.cpp):

#include "test/backend/application.h"
#include "test/gui/application.h"

Expected output:

#include "test/gui/application.h"
#include "test/backend/application.h"

Configuration:

BasedOnStyle: LLVM

I tried 19.1.1 and main (49d48c3).

[llvmbot]

@llvm/issue-subscribers-clang-format

Author: None (EugeneZelenko)

Test case (`test/gui/application.cpp`):

#include "test/backend/application.h"
#include "test/gui/application.h"

Expected output:

#include "test/gui/application.h"
#include "test/backend/application.h"

Configuration:

BasedOnStyle: LLVM

I tried 19.1.1 and main (49d48c3).

[owenca]

This may be related to #118002.