-Wformat in the C++ FE doesn't work as well as it could:
(a) it doesn't report precise locations within the string literal, and
(b) it doesn't underline arguments for those arguments !CAN_HAVE_LOCATION_P,
despite having location wrapper nodes.

For example:

  Wformat-ranges.C:32:10: warning: format '%s' expects argument of type 'char*', but argument 2 has type 'int' [-Wformat=]
  32 |   printf("hello %s", 42);
     |          ^~~~~~~~~~

(a) is due to not wiring up the langhook for extracting substring
    locations.

    This patch uses the one in c-family; it also fixes string literal
    parsing so that it records string concatenations (needed for
    extracting substring locations from concatenated strings).

(b) is due to the call to maybe_constant_value here:
       fargs[j] = maybe_constant_value (argarray[j]);
    within build_over_call.

    The patch fixes this by building a vec of location_t values when
    calling check_function_arguments.
    I attempted to eliminate the maybe_constant_value call here, but
    it's needed by e.g. check_function_sentinel for detecting NULL,
    and that code is in "c-family", so it can't simply call into
    maybe_constant_value (which is in "cp").

With this patch, the output for the above example is improved to:

  Wformat-ranges.C:32:18: warning: format '%s' expects argument of type 'char*', but argument 2 has type 'int' [-Wformat=]
  32 |   printf("hello %s", 42);
     |                 ~^   ~~
     |                  |   |
     |                  |   int
     |                  char*
     |                 %d

gcc/cp/ChangeLog:
	PR c++/56856
	* call.c (build_over_call): Build a vec of locations of the
	arguments before the call to maybe_constant_value, and pass to
	check_function_arguments.
	* cp-lang.c (LANG_HOOKS_GET_SUBSTRING_LOCATION): Define as
	c_get_substring_location.
	* parser.c (cp_parser_string_literal): Capture string
	concatenation locations.

gcc/ChangeLog:
	PR c++/56856
	* input.c (expand_location_to_spelling_point): Add param "aspect"
	and use rather than hardcoding LOCATION_ASPECT_CARET.
	(get_substring_ranges_for_loc): Handle the case of a single token
	within a macro expansion.
	* input.h (expand_location_to_spelling_point): Add "aspect" param,
	defaulting to LOCATION_ASPECT_CARET.

gcc/testsuite/ChangeLog:
	PR c++/56856
	* g++.dg/ext/builtin4.C: Set expected location for warning to the
	correct location within the format string.
	* g++.dg/plugin/plugin.exp (plugin_test_list): Add the plugin and
	files for testing locations within string literal locations from
	the C frontend.
	* g++.dg/warn/Wformat-method.C: New test.
	* g++.dg/warn/Wformat-pr71863.C: New test.
	* g++.dg/warn/Wformat-ranges-c++11.C: New test.
	* g++.dg/warn/Wformat-ranges.C: New test, based on
	gcc.dg/format/diagnostic-ranges.c.
	* gcc.dg/plugin/diagnostic-test-string-literals-1.c
	(test_multitoken_macro): Generalize expected output to work with
	both C and C++.
	* gcc.dg/plugin/diagnostic-test-string-literals-2.c
	(test_stringified_token_1): Likewise.
	(test_stringified_token_3): Likewise.

From-SVN: r264887