[clang-tidy] Don't issue cppcoreguidelines-macro-usage on builtin macros

Before the patch calling clang-tidy with -header-filter=.* -system-headers would
result in a few hundred useless warnings:
  warning: macro '_GNU_SOURCE' used to declare a constant; consider using a 'constexpr' constant [cppcoreguidelines-macro-usage]
  warning: macro '_LP64' used to declare a constant; consider using a 'constexpr' constant [cppcoreguidelines-macro-usage]
  warning: macro '__ATOMIC_ACQUIRE' used to declare a constant; consider using a 'constexpr' constant [cppcoreguidelines-macro-usage]
  warning: macro '__ATOMIC_ACQ_REL' used to declare a constant; consider using a 'constexpr' constant [cppcoreguidelines-macro-usage]
  warning: macro '__ATOMIC_CONSUME' used to declare a constant; consider using a 'constexpr' constant [cppcoreguidelines-macro-usage]
  warning: macro '__ATOMIC_RELAXED' used to declare a constant; consider using a 'constexpr' constant [cppcoreguidelines-macro-usage]
  warning: macro '__ATOMIC_RELEASE' used to declare a constant; consider using a 'constexpr' constant [cppcoreguidelines-macro-usage]
  warning: macro '__ATOMIC_SEQ_CST' used to declare a constant; consider using a 'constexpr' constant [cppcoreguidelines-macro-usage]
  warning: macro '__BIGGEST_ALIGNMENT__' used to declare a constant; consider using a 'constexpr' constant [cppcoreguidelines-macro-usage]
  ... and so on

llvm-svn: 358621