[clang-tidy] llvm-header-guard suggest invalid ifdef macros name

When https://clang.llvm.org/extra/clang-tidy/checks/llvm/header-guard.html check sees a file with has "llvm" inside its filepath (e.g. `clang-tools-extra/clang-tidy/llvm/HeaderGuardCheck.h`), instead of making such `ifdef`:
```cpp
#ifndef LLVM_CLANG_TOOLS_EXTRA_CLANG_TIDY_LLVM_HEADERGUARDCHECK_H
```
The check makes:
```cpp
#ifndef LLVM_HEADERGUARDCHECK_H
```

The check removes leading part if sees "LLVM" in check-name which is incorrect.