libcpp: Fix ICE with -Wtraditional preprocessing [PR101638]

The following testcase ICEs in cpp_sys_macro_p, because cpp_sys_macro_p
is called for a builtin macro which doesn't use node->value.macro union
member but a different one and so dereferencing it ICEs.
As the testcase is distilled from contemporary glibc headers, it means
basically -Wtraditional now ICEs on almost everything.

The fix can be either the patch below, return true for builtin macros,
or we could instead return false for builtin macros, or the fix could
be also (untested):
--- libcpp/expr.c       2021-05-07 10:34:46.345122608 +0200
+++ libcpp/expr.c       2021-08-12 09:54:01.837556365 +0200
@@ -783,13 +783,13 @@ cpp_classify_number (cpp_reader *pfile,

       /* Traditional C only accepted the 'L' suffix.
          Suppress warning about 'LL' with -Wno-long-long.  */
-      if (CPP_WTRADITIONAL (pfile) && ! cpp_sys_macro_p (pfile))
+      if (CPP_WTRADITIONAL (pfile))
        {
          int u_or_i = (result & (CPP_N_UNSIGNED|CPP_N_IMAGINARY));
          int large = (result & CPP_N_WIDTH) == CPP_N_LARGE
                       && CPP_OPTION (pfile, cpp_warn_long_long);

-         if (u_or_i || large)
+         if ((u_or_i || large) && ! cpp_sys_macro_p (pfile))
            cpp_warning_with_line (pfile, large ? CPP_W_LONG_LONG : CPP_W_TRADITIONAL,
                                   virtual_location, 0,
                                   "traditional C rejects the \"%.*s\" suffix",
The builtin macros at least currently don't add any suffixes
or numbers -Wtraditional would like to warn about.  For floating
point suffixes, -Wtraditional calls cpp_sys_macro_p only right
away before emitting the warning, but in the above case the ICE
is because cpp_sys_macro_p is called even if the number doesn't
have any suffixes (that is I think always for builtin macros
right now).

2021-08-12  Jakub Jelinek  <jakub@redhat.com>

	PR preprocessor/101638
	* macro.c (cpp_sys_macro_p): Return true instead of
	crashing on builtin macros.

	* gcc.dg/cpp/pr101638.c: New test.