diagnostics: copy GCC9's format

Currently, when we quote the source file, we indent it with one space,
and preserve tabulations, so there is a discrepancy and the visual
rendering is bad.  One way out is to indent with a tab instead of a
space, but then this space can be used for more information.  This is
what GCC9 does.  Let's play copy cats.

See
https://lists.gnu.org/archive/html/bison-patches/2019-04/msg00025.html
https://developers.redhat.com/blog/2019/03/08/usability-improvements-in-gcc-9/
https://gcc.gnu.org/onlinedocs/gccint/Guidelines-for-Diagnostics.html#Guidelines-for-Diagnostics

* src/location.c (location_caret): Prefix quoted lines with the line
number and a pipe, fitting 8 columns.

* tests/actions.at, tests/c++.at, tests/conflicts.at,
* tests/diagnostics.at, tests/input.at, tests/java.at,
* tests/named-refs.at, tests/reduce.at, tests/regression.at,
* tests/sets.at: Adjust expectations.
Partly by "./build-aux/update-test tests/testsuite.dir/*/testsuite.log"
repeatedly, and partly by hand.

src/location.c

@@ -191,7 +191,7 @@ location_caret (location loc, const char *style, FILE *out)
       {
         /* Quote the file (at most first line in the case of multiline
            location).  Indent by a single column.  */
-        fputc (' ', out);
+        fprintf (out, "%5d | ", loc.start.line);
         bool single_line = loc.start.line == loc.end.line;
         /* Consider that single point location (with equal boundaries)
            actually denote the character that they follow.  */
@@ -215,7 +215,7 @@ location_caret (location loc, const char *style, FILE *out)
 
         {
           /* Print the carets with the same indentation as above.  */
-          fprintf (out, " %*s", loc.start.column - 1, "");
+          fprintf (out, "      | %*s", loc.start.column - 1, "");
           begin_use_class (style, out);
           putc ('^', out);
           /* Underlining a multiline location ends with the first