PR c/81405 identifies a crash when printing fix-it hints from
-Wmissing-braces when there are excess elements.

The fix-it hints are bogus (which I've filed separately as PR c/81432),
but they lead to a crash within the fix-it consolidation logic I added
in r247548, in line_corrections::add_hint.

The root cause is that some of the fix-it hints are out-of-order
with respect to the column numbers they affect, which can lead to negative
values when computing the gap between the fix-it hints, leading to bogus
memcpy calls that generate out-of-bounds buffer accesses.

The fix is to sort the fix-it hints after filtering them, ensuring that
the gap >= 0.  The patch also adds numerous assertions to the code, both
directly, and by moving the memcpy calls and their args behind
interfaces (themselves containing gcc_assert).

This fixes the crash; it doesn't fix the bug in -Wmissing-braces that
leads to the bogus hints.

gcc/ChangeLog:
	PR c/81405
	* diagnostic-show-locus.c (fixit_cmp): New function.
	(layout::layout): Sort m_fixit_hints.
	(column_range::column_range): Assert that the values are valid.
	(struct char_span): New struct.
	(correction::overwrite): New method.
	(struct source_line): New struct.
	(line_corrections::add_hint): Add assertions.  Reimplement memcpy
	calls in terms of classes source_line and char_span, and
	correction::overwrite.
	(selftest::test_overlapped_fixit_printing_2): New function.
	(selftest::diagnostic_show_locus_c_tests): Call it.

gcc/testsuite/ChangeLog:
	PR c/81405
	* gcc.dg/Wmissing-braces-fixits.c: Add coverage for PR c/81405.  */

From-SVN: r250187