Fix issues with unrepresentable column numbers (PR c++/77949)

Fix issues with unrepresentable column numbers (PR c++/77949)

PR c++/77949 identifies an ICE when the C++ frontend attempts to emit a
fix-it hint inserting a missing semicolon at column 4097 of a source file.

This column value exceeds LINE_MAP_MAX_COLUMN_NUMBER and hence isn't
representable using a location_t.

Attempting to do so leads to these problems, which this patch fixes:

(a) when encountering a column number > LINE_MAP_MAX_COLUMN_NUMBER we
create a new linemap with m_column_and_range_bits == 0, but
linemap_position_for_column doesn't check for this, and hence can emit
a bogus location_t value that's calculated relative to the previous
linemap start, but which will be decoded relative to the new linemap,
leading to very large incorrect line values.

(b) when encountering a column number that can't be represented, and
for which the linemap was pre-existing, the code would hit this assertion:
  if (linemap_assert_fails (column < (1u << map->m_column_and_range_bits)))
around a bail-out condition.  The patch replaces this assertion with a
simple conditional, to stop the ICE when this occurs, and fixes the
bit count (effective column bits, vs column+range bits)

(c) the C++ frontend wasn't checking for failure of
linemap_position_for_loc_and_offset when considering emitting the fix-it
hint.  The patch adds a conditional, so that no fix-it hint is emitted
if the location is bogus.

gcc/cp/ChangeLog:
	PR c++/77949
	* parser.c (cp_parser_class_specifier_1): Only suggest inserting
	a missing semicolon if we have a valid insertion location for
	the fix-it hint.

gcc/ChangeLog:
	PR c++/77949
	* input.c (selftest::test_accessing_ordinary_linemaps): Verify
	that we correctly handle column numbers greater than
	LINE_MAP_MAX_COLUMN_NUMBER.

gcc/testsuite/ChangeLog:
	PR c++/77949
	* g++.dg/diagnostic/pr77949.C: New test case.

libcpp/ChangeLog:
	PR c++/77949
	* line-map.c (linemap_position_for_column): When calling
	linemap_start_line, detect if a new linemap was created with
	0 column bits, and bail out early if this is the case.
	(linemap_position_for_loc_and_offset): Replace overzealous
	linemap_assert_fails with a simple conditional; use correct
	bit count.

From-SVN: r244292