With profiledbootstrap and --with-build-config=bootstrap-lto, linemap_add
may create a macro map when we run out of line map space.  This patch
changes start_location to UNKNOWN_LOCATION (0) in this case.

Tested with profiledbootstrap and --with-build-config=bootstrap-lto on
Linux/x86-64.

	PR bootstrap/86872
	* line-map.c (pure_location_p): Return true if linemap_lookup
	returns NULL.
	(linemap_add): Set start_location to 0 if we run out of line map
	space.

From-SVN: r263845