[AST] Store regular ValueDecl* in BindingDecl (NFC)

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

Reviewed By: aaron.ballman

Differential Revision: https://reviews.llvm.org/D99455

clang/include/clang/AST/DeclCXX.h

@@ -3827,7 +3827,7 @@ public:
 /// 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
@@ -3853,7 +3853,7 @@ public:
 
   /// 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.

clang/lib/AST/DeclCXX.cpp

@@ -3179,12 +3179,6 @@ 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)