diagnostics: add -ffixit support for deprecated features

Issue directives for IDE/editors to fix the source file.
http://clang.llvm.org/docs/UsersManual.html#cmdoption-fdiagnostics-parseable-fixits

Do it for deprecated features.  For instance:

    $ cat foo.y
    %error-verbose

    %name-prefix = "foo"
    %name-prefix="bar"
    %define parser_class_name "Parser"

    %%
    exp:;

    $ LC_ALL=C ./_build/8d/tests/bison -ffixit /tmp/foo.yy
    /tmp/foo.yy:1.1-14: warning: deprecated directive, use '%define parse.error verbose' [-Wdeprecated]
     %error-verbose
     ^^^^^^^^^^^^^^
    fix-it:"/tmp/foo.yy":{1:1-1:15}:"%define parse.error verbose"
    /tmp/foo.yy:3.1-20: warning: deprecated directive, use '%define api.prefix {foo}' [-Wdeprecated]
     %name-prefix = "foo"
     ^^^^^^^^^^^^^^^^^^^^
    fix-it:"/tmp/foo.yy":{3:1-3:21}:"%define api.prefix {foo}"
    /tmp/foo.yy:4.1-18: warning: deprecated directive, use '%define api.prefix {bar}' [-Wdeprecated]
     %name-prefix="bar"
     ^^^^^^^^^^^^^^^^^^
    fix-it:"/tmp/foo.yy":{4:1-4:19}:"%define api.prefix {bar}"
    /tmp/foo.yy:5.9-25: warning: deprecated directive, use '%define api.parser.class {Parser}' [-Wdeprecated]
     %define parser_class_name "Parser"
             ^^^^^^^^^^^^^^^^^
    fix-it:"/tmp/foo.yy":{5:9-5:26}:"%define api.parser.class {Parser}"
    /tmp/foo.yy:5.9-25: error: %define variable 'api.parser.class' is not used
     %define parser_class_name "Parser"
             ^^^^^^^^^^^^^^^^^

* src/getargs.h, src/getargs.c (feature_fixit_parsable): New.
(feature_types, feature_args): Use it.
* src/complain.c (deprecated_directive): Use it.

* tests/input.at: Check it.

src/getargs.c

@@ -228,6 +228,7 @@ static const char * const feature_args[] =
 {
   "none",
   "caret", "diagnostics-show-caret",
+  "fixit", "diagnostics-parseable-fixits",
   "all",
   0
 };
@@ -236,6 +237,7 @@ static const int feature_types[] =
 {
   feature_none,
   feature_caret, feature_caret,
+  feature_fixit_parsable, feature_fixit_parsable,
   feature_all
 };