lto: Ensure we force a change for file/line/column after clear_line_info

lto: Ensure we force a change for file/line/column after clear_line_info

As discussed yesterday:
On the streamer out side, we call clear_line_info
in multiple spots which resets the current_* values to something, but on the
reader side, we don't have corresponding resets in the same location, just have
the stream_* static variables that keep the current values through the
entire stream in (so across all the clear_line_info spots in a single LTO
object but also across jumping from one LTO object to another one).
Now, in an earlier version of my patch it actually broke LTO bootstrap
(and a lot of LTO testcases), so for the BLOCK case I've solved it by
clear_line_info setting current_block to something that should never appear,
which means that in the LTO stream after the clear_line_info spots including
the start of the LTO stream we force the block change bit to be set and thus
BLOCK to be streamed and therefore stream_block from earlier to be
ignored.  But for the rest I think that is not the case, so I wonder if we
don't sometimes end up with wrong line/column info because of that, or
please tell me what prevents that.
clear_line_info does:
  ob->current_file = NULL;
  ob->current_line = 0;
  ob->current_col = 0;
  ob->current_sysp = false;
while I think NULL current_file is something that should likely be different
from expanded_location (...).file (UNKNOWN_LOCATION/BUILTINS_LOCATION are
handled separately and not go through the caching), I think line number 0
can sometimes occur and especially column 0 occurs frequently if we ran out
of location_t with columns info.  But then we do:
      bp_pack_value (bp, ob->current_file != xloc.file, 1);
      bp_pack_value (bp, ob->current_line != xloc.line, 1);
      bp_pack_value (bp, ob->current_col != xloc.column, 1);
and stream the details only if the != is true.  If that happens immediately
after clear_line_info and e.g. xloc.column is 0, we would stream 0 bit and
not stream the actual value, so on read-in it would reuse whatever
stream_col etc. were before.  Shouldn't we set some ob->current_* new bit
that would signal we are immediately past clear_line_info which would force
all these != checks to non-zero?  Either by oring something into those
tests, or perhaps:
  if (ob->current_reset)
    {
      if (xloc.file == NULL)
        ob->current_file = "";
      if (xloc.line == 0)
        ob->current_line = 1;
      if (xloc.column == 0)
        ob->current_column = 1;
      ob->current_reset = false;
    }
before doing those bp_pack_value calls with a comment, effectively forcing
all 6 != comparisons to be true?

2020-09-04  Jakub Jelinek  <jakub@redhat.com>

	* lto-streamer.h (struct output_block): Add reset_locus member.
	* lto-streamer-out.c (clear_line_info): Set reset_locus to true.
	(lto_output_location_1): If reset_locus, clear it and ensure
	current_{file,line,col} is different from xloc members.