Use specialized param visit function for params

Use specialized param visit function for params

This commit introduce a new public function to visit function parameters
in the default visitor. It allows visitors derived from DefaultVisitor
to override only a small part of the default visitor.

gcc/rust/ChangeLog:

	* ast/rust-ast-visitor.cc (DefaultASTVisitor::visit_function_params):
	Add specialized function to visit function parameters.
	(DefaultASTVisitor::visit): Remove parameter visit and call specialized
	function instead.
	* ast/rust-ast-visitor.h: Add function prototye.
	* resolve/rust-late-name-resolver-2.0.cc (Late::visit): Remove
	function.
	(Late::visit_function_params): Override specialized visit function.
	* resolve/rust-late-name-resolver-2.0.h: Add overriden function
	prototype.

Signed-off-by: Pierre-Emmanuel Patry <pierre-emmanuel.patry@embecosm.com>