glr.cc, yacc.c: initialize yylloc properly

There are several issues to address here.  One is that yylloc should
be initialized when possible.  Another is that the push parser needs
to update yypushed_loc when the user modified it.  And if the parser
starts by a reduction of an empty, it uses the first location on the
stack, which, therefore, must also be initialized to this initial
location.

This is getting complex, especially since because initializing a
global (impure interface) is different from initializing a local
variable.  To simplify, the local yylloc is not initialized during its
definition.

* data/c.m4 (b4_yyloc_default_define): Replace by...
(b4_yyloc_default): this.
Adjust dependencies.
* data/glr.cc: Initialize yylloc.
* data/yacc.c (b4_declare_scanner_communication_variables):
Initialize yylloc during its definition.
Don't define yyloc_default.
(yypush_parse): The location formal is not const, as we might
initialize it.
(yyparse): Define yyloc_default.
Use it before running the user initial action.
Possibly update the first location on the stack, and the pushed
location after the user initial action.
* tests/actions.at (Initial locations): Check that the initial
location is correct.

data/glr.c

@@ -226,8 +226,8 @@ b4_percent_code_get([[top]])[
    right-hand sides.  Unlike the standard yacc.c template, here we set
    the default value of $$ to a zeroed-out value.  Since the default
    value is undefined, this behavior is technically correct.  */
-static YYSTYPE yyval_default;]b4_locations_if([
-b4_yyloc_default_define])[
+static YYSTYPE yyval_default;]b4_locations_if([[
+static YYLTYPE yyloc_default][]b4_yyloc_default;])[
 
 /* Copy the second part of user declarations.  */
 ]b4_user_post_prologue