diagnostics: prefer ^~~~ to ^^^^ to underline code

That's what both GCC and Clang do, and it is indeed much nicer to
read.  From:

    foo.y:1.1-14: warning: deprecated directive, use '%define parse.error verbose' [-Wdeprecated]
     %error-verbose
     ^^^^^^^^^^^^^^
    foo.y:4.1-20: warning: deprecated directive, use '%define api.prefix {foo}' [-Wdeprecated]
     %name-prefix = "foo"
     ^^^^^^^^^^^^^^^^^^^^

to:

    foo.y:1.1-14: warning: deprecated directive, use '%define parse.error verbose' [-Wdeprecated]
     %error-verbose
     ^~~~~~~~~~~~~~
    foo.y:4.1-20: warning: deprecated directive, use '%define api.prefix {foo}' [-Wdeprecated]
     %name-prefix = "foo"
     ^~~~~~~~~~~~~~~~~~~~

* src/location.c (location_caret): Use ^~~~.
Adjust tests expectations.

src/location.c

@@ -209,7 +209,7 @@ location_caret (location loc, FILE *out)
           /* Print the carets (at least one), with the same indent as above.*/
           fprintf (out, " %*s", loc.start.column - 1, "");
           for (i = loc.start.column; i == loc.start.column || i < len; ++i)
-            putc ('^', out);
+            putc (i == loc.start.column ? '^' : '~', out);
           }
         putc ('\n', out);
       }