[AST] Store regular ValueDecl* in BindingDecl

[aaronpuchert]

Summary

We were always storing a regular ValueDecl* as decomposition declaration
and haven't been using the opportunity to initialize it lazily.

[aaronpuchert]

Ping @rsmith.

[aaronpuchert]

Ping @rsmith.

[aaron.ballman]

LGTM, please add NFC to the commit message though. Thanks!

clang/include/clang/AST/DeclCXX.h

 ///   int n[3];
 ///   auto &[a, b, c] = n;
 ///
 /// a, b, and c are BindingDecls, whose bindings are the expressions
 /// x[0], x[1], and x[2] respectively, where x is the implicit
 /// DecompositionDecl of type 'int (&)[3]'.
 class BindingDecl : public ValueDecl {
   /// The declaration that this binding binds to part of.
-  LazyDeclPtr Decomp;
+  ValueDecl *Decomp;
   /// The binding represented by this declaration. References to this
   /// declaration are effectively equivalent to this expression (except
   /// that it is only evaluated once at the point of declaration of the
   /// binding).
   Expr *Binding = nullptr;
 
   BindingDecl(DeclContext *DC, SourceLocation IdLoc, IdentifierInfo *Id)
       : ValueDecl(Decl::Binding, DC, IdLoc, Id, QualType()) {}
public:
 
   /// Get the expression to which this declaration is bound. This may be null
   /// in two different cases: while parsing the initializer for the
   /// decomposition declaration, and when the initializer is type-dependent.
   Expr *getBinding() const { return Binding; }
 
   /// Get the decomposition declaration that this binding represents a
   /// decomposition of.
-  ValueDecl *getDecomposedDecl() const;
+  ValueDecl *getDecomposedDecl() const { return Decomp; }
 
   /// Get the variable (if any) that holds the value of evaluating the binding.
   /// Only present for user-defined bindings for tuple-like types.
   VarDecl *getHoldingVar() const;
 
   /// Set the binding for this BindingDecl, along with its declared type (which
   /// should be a possibly-cv-qualified form of the type of the binding, or a
   /// reference to such a type).

clang/lib/AST/DeclCXX.cpp

BindingDecl *BindingDecl::Create(ASTContext &C, DeclContext *DC,
                                  SourceLocation IdLoc, IdentifierInfo *Id) {
   return new (C, DC) BindingDecl(DC, IdLoc, Id);
 }
 
 BindingDecl *BindingDecl::CreateDeserialized(ASTContext &C, unsigned ID) {
   return new (C, ID) BindingDecl(nullptr, SourceLocation(), nullptr);
 }
 
-ValueDecl *BindingDecl::getDecomposedDecl() const {
-  ExternalASTSource *Source =
-      Decomp.isOffset() ? getASTContext().getExternalSource() : nullptr;
-  return cast_or_null<ValueDecl>(Decomp.get(Source));
-}
-
 VarDecl *BindingDecl::getHoldingVar() const {
   Expr *B = getBinding();
   if (!B)
     return nullptr;
   auto *DRE = dyn_cast<DeclRefExpr>(B->IgnoreImplicit());
   if (!DRE)
     return nullptr;