compiler: reword "declared and not used" error message

This is a gofrontend copy of https://golang.org/cl/203282.

From the CL 203282 description:

    "declared and not used" is technically correct, but might confuse
    the user. Switching "and" to "but" will hopefully create the
    contrast for the users: they did one thing (declaration), but
    not the other --- actually using the variable.

Change-Id: Ie302171c3c6aa6d4429dc125cf994be8ad58554a
Reviewed-on: https://go-review.googlesource.com/c/gofrontend/+/273629
Trust: Ian Lance Taylor <iant@golang.org>
Reviewed-by: Than McIntosh <thanm@google.com>
diff --git a/go/gogo.cc b/go/gogo.cc
index 93a4a57..a5e4521 100644
--- a/go/gogo.cc
+++ b/go/gogo.cc
@@ -3744,7 +3744,7 @@
           && !var->type()->is_error()
           && (init == NULL || !init->is_error_expression())
           && !Lex::is_invalid_identifier(named_object->name()))
-	go_error_at(var->location(), "%qs declared and not used",
+	go_error_at(var->location(), "%qs declared but not used",
 		    named_object->message_name().c_str());
     }
   return TRAVERSE_CONTINUE;
diff --git a/go/parse.cc b/go/parse.cc
index 00ac2f8..b1925ed 100644
--- a/go/parse.cc
+++ b/go/parse.cc
@@ -4817,7 +4817,7 @@
 	    }
 	}
       if (!used)
-	go_error_at(type_switch.location, "%qs declared and not used",
+	go_error_at(type_switch.location, "%qs declared but not used",
 		    Gogo::message_name(var_name).c_str());
     }
   return statement;