ast: Implement reconstruct() logic for Pattern nodes

[Pasta-coder]

This patch implements the reconstruct() pattern for the Pattern AST hierarchy. This is the AST reconstruction refactor (following Types and Paths in #3799 and #4395 ), 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.
• Updating derive(PartialEq) expansion logic to use reconstruct()
  instead of rebuilding paths manually.

gcc/rust/ChangeLog:

* expand/rust-derive-cmp-common.cc (EnumMatchBuilder::tuple): Use
reconstruct() to clone variant paths.
(EnumMatchBuilder::strukt): Likewise. 
* ast/rust-ast.h (Pattern::reconstruct): New public API.
(Pattern::reconstruct_impl): New pure virtual method.
* 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.