[libc++] Add missing %{flags} substitution to clang-tidy (#171689)

Flags that should be used both for compiling and for linking are
provided through the %{flags} substitution. Our clang-tidy tests should
be using them, not only %{compile_flags}.

NOKEYCHECK=True
GitOrigin-RevId: a04627621efcb6a18fc6c32a4f70a45a71875cfd