[clang-tidy] Fix pr48613: "llvm-header-guard uses a reserved identifier"

Fixes https://bugs.llvm.org/show_bug.cgi?id=48613.

llvm-header-guard is suggesting header guards with leading underscores
if the header file path begins with a '/' or similar special character.
Only reserved identifiers should begin with an underscore.

Differential Revision: https://reviews.llvm.org/D114149