gccrs: expand: Add new ExpandVisitor class

gccrs: expand: Add new ExpandVisitor class

This class takes care of actually performing the macro expansion by calling
into the MacroExpander for each node of a given AST, leaving the job of
cfg-stripping nodes to the AttrVisitor.

gcc/rust/ChangeLog:

	* Make-lang.in: Add new object file.
	* expand/rust-attribute-visitor.cc (AttrVisitor::go): Visit all items of a
	crate.
	(AttrVisitor::expand_struct_fields): Do not perform macro expansion anymore.
	(AttrVisitor::expand_function_params): Likewise.
	(AttrVisitor::expand_generic_args): Likewise.
	(AttrVisitor::expand_qualified_path_type): Likewise.
	(AttrVisitor::expand_self_param): Likewise.
	(AttrVisitor::expand_trait_function_decl): Likewise.
	(AttrVisitor::expand_trait_method_decl): Likewise.
	(AttrVisitor::visit): Likewise.
	(AttrVisitor::maybe_expand_expr): Remove function.
	(AttrVisitor::maybe_expand_type): Likewise.
	* expand/rust-attribute-visitor.h: Do not keep MacroExpander inside AttrVisitor
	anymore.
	* expand/rust-macro-expand.h (struct MacroExpander): Turn ContextType into an
	enum class for more type safety.
	* expand/rust-macro-expand.cc (MacroExpander::expand_crate): Use new ContextType
	API.
	* rust-session-manager.cc (Session::expansion): Call into ExpandVisitor.
	* expand/rust-expand-visitor.cc: New file.
	* expand/rust-expand-visitor.h: New file.

gcc/testsuite/ChangeLog:

	* rust/compile/macro49.rs: New test.
	* rust/compile/macro50.rs: New test.