gccrs: Add AST::AltPattern class

gccrs: Add AST::AltPattern class

gcc/rust/ChangeLog:

	* ast/rust-ast-dump.cc
	(Dump::visit): Add AltPattern visitor.
	* ast/rust-ast-dump.h:
	(Dump::visit): Add AltPattern visitor.
	* ast/rust-ast-full-decls.h
	(class AltPattern): Add declaration.
	* ast/rust-ast-visitor.h:
	(ASTVisitor::visit): Add AltPattern visitor.
	* ast/rust-ast.cc
	(AltPattern::as_string): Add definition.
	(AltPattern::accept_vis): Add definition.
	* ast/rust-pattern.h
	(class AltPattern): Add declaration.
	* checks/errors/rust-feature-gate.h:
	(FeatureGate::visit) Add AltPattern visitor
	* expand/rust-attribute-visitor.cc
	(AttrVisitor::visit): Add AltPattern visitor.
	* expand/rust-attribute-visitor.h:
	(AttrVisitor::visit): Add AltPattern visitor.
	* hir/rust-ast-lower-base.cc
	(ASTLoweringBase::visit): Add AltPattern visitor.
	* hir/rust-ast-lower-base.h:
	(ASTLoweringBase::visit): Add AltPattern visitor.
	* resolve/rust-ast-resolve-base.cc
	(ResolverBase::visit): Add AltPattern visitor.
	* resolve/rust-ast-resolve-base.h:
	(ResolverBase::visit): Add AltPattern visitor.
	* resolve/rust-early-name-resolver.cc
	(EarlyNameResolver::visit): Add AltPattern visitor.
	* resolve/rust-early-name-resolver.h:
	(EarlyNameResolver::visit): Add AltPattern visitor.
	* util/rust-attributes.cc
	(AttributeChecker::visit): Add AltPattern visitor.
	* util/rust-attributes.h:
	(AttributeChecker::visit): Add AltPattern visitor.

Signed-off-by: Owen Avery <powerboat9.gamer@gmail.com>