Add warning flag -Wordered-compare-function-pointers.

[efriedma]

Summary

The C standard doesn't allow comparisons like "f1 < f2" (where f1 and f2 are function pointers), but we allow them as an extension. Add a warning flag to control this warning.

(Not sure I like the name, but this seems to describe the warning reasonably well. Suggestions welcome.)

[lebedev.ri]

Ideally there should be a test that verifies that -Wordered-compare-function-pointers / -Wno-ordered-compare-function-pointers / the default is what you expect it to be.

include/clang/Basic/DiagnosticSemaKinds.td

def err_typecheck_sub_ptr_compatible : Error<
   "pointers to incompatible types}0,1">;
 def ext_typecheck_ordered_comparison_of_pointer_integer : ExtWarn<
   "ordered comparison between pointer and integer (%0 and %1)">;
 def ext_typecheck_ordered_comparison_of_pointer_and_zero : Extension<
   "ordered comparison between pointer and zero (%0 and %1) is an extension">;
 def err_typecheck_ordered_comparison_of_pointer_and_zero : Error<
   "ordered comparison between pointer and zero (%0 and %1)">;
 def ext_typecheck_ordered_comparison_of_function_pointers : ExtWarn<
-  "ordered comparison of function pointers (%0 and %1)">;
+  "ordered comparison of function pointers (%0 and %1)">,
+  InGroup<DiagGroup<"ordered-compare-function-pointers">>;
 def ext_typecheck_comparison_of_fptr_to_void : Extension<
   "equality comparison between function pointer and void pointer (%0 and %1)">;
 def err_typecheck_comparison_of_fptr_to_void : Error<
   "equality comparison between function pointer and void pointer (%0 and %1)">;
 def ext_typecheck_comparison_of_pointer_integer : ExtWarn<
   "comparison between pointer and integer (%0 and %1)">;
 def err_typecheck_comparison_of_pointer_integer : Error<
   "comparison between pointer and integer (%0 and %1)">;

test/Misc/warning-flags.c

(2) It prevents us adding new warnings to Clang that have no -W flag.  All
     new warnings should have -W flags.
 
     If you add a new warning without a flag, this test will fail.  To fix
     this test, simply add a warning group to that warning.
 
 
 The list of warnings below should NEVER grow.  It should gradually shrink to 0.
 
-CHECK: Warnings without flags (77):
+CHECK: Warnings without flags (76):
 CHECK-NEXT:   ext_excess_initializers
 CHECK-NEXT:   ext_excess_initializers_in_char_array_initializer
 CHECK-NEXT:   ext_expected_semi_decl_list
 CHECK-NEXT:   ext_explicit_specialization_storage_class
 CHECK-NEXT:   ext_initializer_string_for_char_array_too_long
 CHECK-NEXT:   ext_missing_declspec
 CHECK-NEXT:   ext_missing_whitespace_after_macro_name
 CHECK-NEXT:   ext_new_paren_array_nonconst
 CHECK-NEXT:   ext_plain_complex
 CHECK-NEXT:   ext_template_arg_extra_parens
 CHECK-NEXT:   ext_typecheck_comparison_of_pointer_integer
 CHECK-NEXT:   ext_typecheck_cond_incompatible_operands
-CHECK-NEXT:   ext_typecheck_ordered_comparison_of_function_pointers
 CHECK-NEXT:   ext_typecheck_ordered_comparison_of_pointer_integer
 CHECK-NEXT:   ext_using_undefined_std
 CHECK-NEXT:   pp_invalid_string_literal
 CHECK-NEXT:   pp_out_of_date_dependency
 CHECK-NEXT:   pp_poisoning_existing_macro
 CHECK-NEXT:   warn_accessor_property_type_mismatch
 CHECK-NEXT:   warn_arcmt_nsalloc_realloc
 CHECK-NEXT:   warn_asm_label_on_auto_decl