gccrs: expand: Add handling for macro expansion in pattern context

gccrs: expand: Add handling for macro expansion in pattern context

gcc/rust/ChangeLog:

	* ast/rust-ast-fragment.cc (Fragment::is_pattern_fragment): New function.
	(Fragment::take_pattern_fragment): Likewise.
	(Fragment::assert_single_fragment): Likewise.
	* ast/rust-ast-fragment.h: Declare them.
	* ast/rust-ast.cc (SingleASTNode::SingleASTNode): Add new constructor for pattern
	single AST nodes.
	(SingleASTNode::operator=): Handle patterns.
	(SingleASTNode::accept_vis): Likewise.
	(SingleASTNode::is_error): Likewise.
	(SingleASTNode::as_string): Likewise.
	* ast/rust-ast.h: Add get_pattern_ptr() functions.
	* ast/rust-expr.h: Likewise.
	* ast/rust-item.h: Likewise.
	* ast/rust-pattern.h: Likewise.
	* ast/rust-stmt.h: Likewise.
	* expand/rust-expand-visitor.cc (derive_item): Use new API enum values.
	(expand_item_attribute): Likewise.
	(expand_stmt_attribute): Likewise.
	(ExpandVisitor::maybe_expand_pattern): New function.
	(ExpandVisitor::expand_closure_params): Handle patterns.
	(ExpandVisitor::visit): Add new visitors for patterns and missed exprs.
	* expand/rust-expand-visitor.h: Declare them.
	* expand/rust-macro-expand.cc (transcribe_pattern): New function.
	(transcribe_context): Call it.
	* expand/rust-macro-expand.h (struct MacroExpander): New Context type.

gcc/testsuite/ChangeLog:

	* rust/compile/issue-3726.rs: New test.
	* rust/compile/issue-3898.rs: New test.