In clang tidy, the check [llvm-header-guard] looks for LLVM style header guards, but I can't find any examples of proper LLVM header guard style, specifically the structure of the name given to the define, the coding standards pages does not mention anything.
Looking at the unit tests:
it seems to accept a few variations on the commonly used patterns. For a file named include/llvm/ADT/foo.h
the convention seems to be:
#ifndef LLVM_ADT_FOO_H
#define LLVM_ADT_FOO_H
//...
#endif // LLVM_ADT_FOO_H