In r253096
  ("C++: underline parameters in mismatching function calls"
  aka 5d78d423a5f7a1d135c7bb678e82007678d1313c
    https://gcc.gnu.org/ml/gcc-patches/2017-09/msg01546.html )
I updated the C++ FE's handling of mismatched types in function calls
so that it underlines the pertinent param of the callee, rather than
just the function name.

The following patch does the same for the C frontend.

Given e.g. this type mismatch:

  extern int callee (int one, const char *two, float three);

  int caller (int first, int second, float third)
  {
    return callee (first, second, third);
  }

the C FE currently emits (trunk):

  test.c: In function 'caller':
  test.c:5:25: warning: passing argument 2 of 'callee' makes pointer from
  integer without a cast [-Wint-conversion]
     return callee (first, second, third);
                           ^~~~~~
  test.c:1:12: note: expected 'const char *' but argument is of type 'int'
   extern int callee (int one, const char *two, float three);
              ^~~~~~

whereas with this patch the note underlines the pertinent param of
the callee:

  test.c: In function 'caller':
  test.c:5:25: warning: passing argument 2 of 'callee' makes pointer from
  integer without a cast [-Wint-conversion]
     return callee (first, second, third);
                           ^~~~~~
  test.c:1:41: note: expected 'const char *' but argument is of type 'int'
   extern int callee (int one, const char *two, float three);
                               ~~~~~~~~~~~~^~~

making the problem more obvious to the user.

As with the C++ patch, the patch:

(a) updates the locations of the params to cover the range of all
of their tokens, putting the caret on the first character of the
param name (if present), otherwise at the start of the first token
(doing so requires adding a last_token_location to the c_parser, so
we can determine the location of the last consumed token).

(b) updates the "note" to use the param location, rather than the
fndecl location

gcc/c/ChangeLog:
	* c-decl.c (push_parm_decl): Store c_parm's location into the
	PARAM_DECL.
	(build_c_parm): Add "loc" param and store it within the c_parm.
	* c-parser.c (struct c_parser): Add "last_token_location" field.
	(c_parser_consume_token): Store location of the token into the
	new field.
	(c_parser_declaration_or_fndef): Store params into DECL_ARGUMENTS
	when handling a FUNCTION_DECL, if it doesn't already have them.
	(c_parser_parameter_declaration): Generate a location for the
	parameter, and pass it to the call to build_c_parm.
	* c-tree.h (struct c_parm): Add field "loc".
	(build_c_parm): Add location_t param.
	* c-typeck.c (get_fndecl_argument_location): New function.
	(inform_for_arg): New function.
	(convert_for_assignment): Use inform_for_arg when dealing with
	ic_argpass.

gcc/testsuite/ChangeLog:
	* gcc.dg/diagnostic-range-bad-called-object.c: Update expected
	underlining for param.
	* gcc.dg/param-type-mismatch.c: Update expected results to reflect
	highlighting of parameters; add test coverage for trivial
	parameter decls, and for callback parameters.
	* gcc.dg/pr68533.c: Update location of two errors to reflect
	location of params.

From-SVN: r253411