Cleanup void and variadic parameter type interfaces
    
    While working on something else, it appeared that renaming
    abigail::environment::get_void_type_decl() to
    abigail::environment::get_void_type() and
    abigail::environment::get_variadic_parameter_type_decl() to
    abigail::environment::get_variadic_parameter_type() would be an
    improvement.
    
    This patch implements that.
    
    	* include/abg-ir.h (environment::{get_void_type,
    	get_variadic_parameter_type}): Renamed get_void_type_decl and
    	get_variadic_parameter_type_decl to these.
    	(environment::is_void_type): Remove the overload that takes a bare
    	pointer.
    	(environment::is_variadic_parameter_type): Declare new member
    	function.
    	* src/abg-ir.cc (environment::void_type_): Renamed the data member
    	void_type_decl_ into this.
    	(environment::variadic_marker_type_): Renamed the data member
    	variadic_marker_type_decl_ into this.
    	(environment::{get_void_type, get_variadic_parameter_type}):
    	Renamed get_void_type_decl and get_variadic_parameter_type_decl to
    	these.
    	(environment::is_void_type): Take a smart pointer now.
    	(environment::is_variadic_parameter_type): Define new member
    	function.
    	(synthesize_function_type_from_translation_unit): Adjust.
    	(function_decl::parameter::get_pretty_representation): Likewise.
    	* src/abg-comparison.cc (is_diff_of_variadic_parameter_type):
    	Adjust.
    	* src/abg-dwarf-reader.cc (build_function_type)
    	(build_ir_node_for_void_type): Likewise.
    	* src/abg-reader.cc (build_function_parameter)
    	(build_function_decl, build_function_type): Likewise.
    
    Signed-off-by: Dodji Seketeli <dodji@redhat.com>