ast: Implement reconstruct() logic for Pattern nodes

This patch implements the `reconstruct()` pattern for the `Pattern` AST
hierarchy. This is Phase 3 of the AST reconstruction refactor (following
Types and Paths), designed to allow deep copying of AST nodes while
guaranteeing the generation of fresh NodeIDs.

This is essential for macro expansion (specifically `derive` macros),
where patterns must be duplicated without sharing NodeIDs to avoid
resolution collisions.

Key changes include:
- Implementing `reconstruct_impl` for all `Pattern` subclasses.
- Adding `reconstruct` support to helper classes (`RangePatternBound`,
  `StructPatternField`, `PatternItems`).
- Implementing typed `reconstruct()` wrappers for item families
  (`TupleStructItems`, `TuplePatternItems`, `SlicePatternItems`) to
  ensure type safety and avoid manual downcasting.
- Adding null-safety checks in copy constructors and reconstruction
  logic to handle optional fields (e.g., open-ended ranges or stripped
  nodes) without crashing.
- Integrating with `PathInExpression::reconstruct()` for struct patterns.

gcc/rust/ChangeLog:

	* ast/rust-pattern.h (Pattern::reconstruct_impl): New virtual method.
	(LiteralPattern::reconstruct_impl): Implement reconstruction.
	(IdentifierPattern::reconstruct_impl): Likewise.
	(WildcardPattern::reconstruct_impl): Likewise.
	(RestPattern::reconstruct_impl): Likewise.
	(RangePatternBound::reconstruct): New method.
	(RangePatternBound::reconstruct_impl): New virtual method.
	(RangePatternBoundLiteral::reconstruct_impl): Implement reconstruction.
	(RangePatternBoundPath::reconstruct_impl): Likewise.
	(RangePatternBoundQualPath::reconstruct_impl): Likewise.
	(RangePattern::RangePattern): Fix copy constructor null safety.
	(RangePattern::reconstruct_impl): Implement reconstruction with guards.
	(ReferencePattern::ReferencePattern): Fix copy constructor null safety.
	(ReferencePattern::reconstruct_impl): Implement reconstruction.
	(StructPatternField::reconstruct): New method.
	(StructPatternField::reconstruct_impl): New virtual method.
	(StructPatternField::operator=): Default the assignment operator.
	(StructPatternFieldTuplePat::reconstruct_impl): Implement reconstruction.
	(StructPatternFieldIdentPat::reconstruct_impl): Likewise.
	(StructPatternFieldIdent::reconstruct_impl): Likewise.
	(StructPatternElements::reconstruct): New method.
	(StructPattern::reconstruct_impl): Implement reconstruction using path reconstruct.
	(PatternItems::reconstruct): New protected method.
	(PatternItems::reconstruct_impl): New virtual method.
	(TupleStructItems::reconstruct): New typed reconstruction method.
	(TupleStructItems::reconstruct_impl): New pure virtual implementation.
	(TupleStructItemsNoRest::reconstruct_impl): Implement reconstruction.
	(TupleStructItemsHasRest::reconstruct_impl): Likewise.
	(TupleStructPattern::reconstruct_impl): Implement reconstruction.
	(TuplePatternItems::reconstruct): New typed reconstruction method.
	(TuplePatternItems::reconstruct_impl): New pure virtual implementation.
	(TuplePatternItemsNoRest::reconstruct_impl): Implement reconstruction.
	(TuplePatternItemsHasRest::reconstruct_impl): Likewise.
	(TuplePattern::reconstruct_impl): Implement reconstruction.
	(GroupedPattern::reconstruct_impl): Likewise.
	(SlicePatternItems::reconstruct): New typed reconstruction method.
	(SlicePatternItems::reconstruct_impl): New pure virtual implementation.
	(SlicePatternItemsNoRest::reconstruct_impl): Implement reconstruction.
	(SlicePatternItemsHasRest::reconstruct_impl): Likewise.
	(SlicePattern::reconstruct_impl): Implement reconstruction.
	(AltPattern::reconstruct_impl): Likewise.

Signed-off-by: Jayant Chauhan <0001jayant@gmail.com>

Author: Pasta-coder