This patch is based on grepping the C++ frontend for %P
i.e. diagnostics that refer to a parameter number.  It fixes up
these diagnostics to highlight the pertinent param where appropriate
(and possible), along with various other tweaks, as described in the
ChangeLog.

gcc/cp/ChangeLog:
	PR c++/85110
	* call.c (conversion_null_warnings): Try to use the location of
	the expression for the warnings.  Add notes showing the parameter
	of the function decl, where available.
	(get_fndecl_argument_location): For implicitly-declared functions,
	use the fndecl location rather than that of the param.
	(maybe_inform_about_fndecl_for_bogus_argument_init): New function.
	(convert_like_real): Use it in various places to avoid repetition.
	(complain_about_bad_argument): Likewise.
	* cp-tree.h (maybe_inform_about_fndecl_for_bogus_argument_init):
	New declaration.
	* decl2.c (check_default_args): Put all diagnostics for the fndecl
	into a diagnostic group.  Use the location of the parameter when
	complaining about parameters with missing default arguments in
	preference to that of the fndecl.  Attempt to record the location
	of the first parameter with a default argument and emit a note
	for the first parameter that's missing one.
	* typeck.c (convert_arguments): When complaining about parameters
	with incomplete types, attempt to use the location of the
	argument.  Where available, add a note showing the pertinent
	parameter in the fndecl.
	(convert_for_assignment): When complaining about bad conversions
	at function calls, use the location of the unstripped argument.
	(convert_for_initialization): When checking for bogus references,
	add an auto_diagnostic_group, and update the note to use the
	location of the pertinent parameter, rather than just the callee.

gcc/testsuite/ChangeLog:
	PR c++/85110
	* g++.dg/diagnostic/missing-default-args.C: New test.
	* g++.dg/diagnostic/param-type-mismatch-3.C: New test.
	* g++.dg/diagnostic/param-type-mismatch.C: Add tests for invalid
	references and incomplete types.
	* g++.dg/warn/Wconversion-null-4.C: New test.

From-SVN: r266856