Add AltPattern HIR node

Add AltPattern HIR node

Example:
	match x {
	    0 | 1 => true,
	    _ => false
	}

gcc/rust/ChangeLog:

	* backend/rust-compile-fnparam.h:
	(CompileFnParam::visit): Add AltPattern visitor.
	* backend/rust-compile-pattern.h:
	(CompilePatternCaseLabelExpr::visit): Add AltPattern visitor.
	(CompilePatternBindings::visit): Add AltPattern visitor.
	(CompilePatternLet::visit): Add AltPattern visitor.
	* backend/rust-compile-resolve-path.h:
	(ResolvePathRef::visit): Add AltPattern visitor.
	* backend/rust-compile-var-decl.h:
	(CompileVarDecl::visit): Add AltPattern visitor.
	* checks/errors/rust-const-checker.cc
	(ConstChecker::visit): Add AltPattern visitor.
	* checks/errors/rust-const-checker.h:
	(ConstChecker::visit): Add AltPattern visitor.
	* checks/errors/rust-unsafe-checker.cc
	(UnsafeChecker::visit): Add AltPattern visitor.
	* checks/errors/rust-unsafe-checker.h:
	(UnsafeChecker::visit): Add AltPattern visitor.
	* hir/rust-hir-dump.cc
	(Dump::visit): Add AltPattern visitor.
	* hir/rust-hir-dump.h:
	(Dump::visit): Add AltPattern visitor.
	* hir/tree/rust-hir-full-decls.h
	(class AltPattern): Add forward declaration.
	* hir/tree/rust-hir-pattern.h
	(class AltPattern): New class.
	* hir/tree/rust-hir-visitor.h:
	(HIRFullVisitor::visit): Add AltPattern visitor.
	(HIRFullVisitorBase::visit): Add AltPattern visitor.
	(HIRPatternVisitor::visit): Add AltPattern visitor.
	* hir/tree/rust-hir.h:
	(Pattern::PatternType::ALT): New enumerator.
	* typecheck/rust-hir-type-check-pattern.cc
	(TypeCheckPattern::visit): Add AltPattern visitor.
	* typecheck/rust-hir-type-check-pattern.h:
	(TypeCheckPattern::visit): Add AltPattern visitor.

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