Fix multiple issues with variadic representation

Fix multiple issues with variadic representation

The new variadic representation has introduced multiple issues and ICE
into the codebase. Some early passes in the compiler depend on the
parameters all having a type and being an actual parameter.

gcc/rust/ChangeLog:

	* ast/rust-ast.cc (ExternalFunctionItem::as_string): Adapt as_string
	function to the new ast representation.
	(NamedFunctionParam::as_string): Likewise.
	* ast/rust-item.h: Add a function to test whether a FunctionParam has
	a name pattern.
	* expand/rust-cfg-strip.cc (CfgStrip::visit): Adapt cfg strip visitor
	for the new variadic arguments.
	* hir/rust-ast-lower-extern.h: Adapt lowering to the new variadic
	function representation.
	* metadata/rust-export-metadata.cc (ExportContext::emit_function):
	Change call to constructor.
	* parse/rust-parse-impl.h (Parser::parse_named_function_param): Change
	NamedFunctionParam parsing to accomodate new variadic representation.
	(Parser::parse_external_item): Change external item parsing to use the
	new NamedFunctionParam variadics.
	* parse/rust-parse.h: Add new parsing function prototypes.
	* ast/rust-ast-collector.cc (TokenCollector::visit): Rework token
	collection to take into account variadic parameters.
	* ast/rust-ast-visitor.cc: Likewise.
	* expand/rust-expand-visitor.cc (ExpandVisitor::visit): Change function
	bound to avoid getting the type of a variadic parameter.
	* resolve/rust-ast-resolve-item.cc (ResolveExternItem::visit):
	Likewise.
	* resolve/rust-early-name-resolver.cc (EarlyNameResolver::visit):
	Likewise.

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