[Clang] Add non static data member initializer for BindingDecl member Decomp (llvm#134969)

Static analysis flagged BindingDecl Decomp member as not being
initialized during construction or by any other member function.

Fix is add a default initializer for it like the other member Binding.

Author: shafik

clang/include/clang/AST/DeclCXX.h

@@ -4166,7 +4166,7 @@ class StaticAssertDecl : public Decl {
 /// DecompositionDecl of type 'int (&)[3]'.
 class BindingDecl : public ValueDecl {
   /// The declaration that this binding binds to part of.
-  ValueDecl *Decomp;
+  ValueDecl *Decomp = nullptr;
   /// 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