When the input file contains lone CRs (aka, ^M, \r), the locations see
a new line. Diagnostics look only at \n as end-of-line, so sometimes
there is an offset in diagnostics. Worse yet: sometimes we loop
endlessly waiting for \n to come from a continuous stream of EOF.
Fix that:
- check for EOF
- beware not to call end_use_class if begin_use_class was not
called (which would abort). This could happen if the actual
line is shorter that the expected one.
Prompted by a (private) report from Marc Schönefeld.
* src/location.c (location_caret): here.
* tests/diagnostics.at (Carriage return): New.
d120a07e6b