libstdc++: The Trouble with Tribbles

libstdc++: The Trouble with Tribbles

Fix digit grouping for integers formatted with "{:#Lx}" which were
including the "0x" prefix in the grouped digits. This resulted in output
like "0,xff,fff" instead of "0xff,fff".

Also change std:::basic_format_parse_context to not throw for an arg-id
that is larger than the actual number of format arguments. I clarified
with Victor Zverovich that this is the intended behaviour for the
run-time format-string checks. An out-of-range arg-id should be
diagnosed at compile-time (as clarified by LWG 3825) but not run-time.
The formatting function will still throw at run-time when args.arg(id)
returns an empty basic_format_arg.

libstdc++-v3/ChangeLog:

	* include/std/format (basic_format_parse_context::next_arg_id):
	Only check arg-id is in range during constant evaluation.
	* testsuite/std/format/functions/format.cc: Check "{:#Lx}".
	* testsuite/std/format/parse_ctx.cc: Adjust expected results for
	format-strings using an out-of-range arg-id.