Handle lines encoded into several maps in linemap_position_for_loc_and_offset

Handle lines encoded into several maps in linemap_position_for_loc_and_offset

linemap_position_for_loc_and_offset() tries to generate a location_t
encoding a column offset from the current location, for example, point
to a certain character inside a string. This is trivial to do when the
new location "fits within" the map of the original location. However,
it may happen that the (long) line corresponding to the original
location is encoded in several maps, thus the new location should
actually be encoded in a subsequent map from the original location.
This patch detects this case and adjusts the map correspondingly.

(This shows that the line-map representation is quite wasteful in this
case, because line-maps always start at column 0. That is, map[0]
highest location may encode up to line 8 column 80, then
map[1]->start_location starts encoding at line 8 column 0. Thus, there
are two location_t values that point to the same source location.)

libcpp/ChangeLog:

2015-09-21  Manuel López-Ibáñez  <manu@gcc.gnu.org>

	PR c/66415
	* line-map.c (linemap_position_for_loc_and_offset): Handle the
	case of long lines encoded in multiple maps.

gcc/testsuite/ChangeLog:

2015-09-21  Manuel López-Ibáñez  <manu@gcc.gnu.org>

	PR c/66415
	* gcc.dg/cpp/pr66415-1.c: Test column number.

From-SVN: r227975