Enable/update legacy linespecs in MI.

Enable/update legacy linespecs in MI.

MI is currently using string_to_event_location to enable the use of legacy
linespecs, but using this function (until this patchset) had the (as yet
unnoticed) side effect of allowing both MI and CLI representation for
explicit locations.

This patch simply changes MI to use the same legacy linespec functions
that the python and guile interpreters use.  This eliminates the CLI syntax
for explicit locations (in MI).

gdb/ChangeLog

	* mi/mi-cmd-break.c (mi_cmd_break_insert_1): Use
	string_to_event_location_basic instead of string_to_event_location.