[Clang][Interp] Visit DecompositionDecl and create a local variable (#100400)

Summary:
The following code should be well-formed:
```C++
float decompose_complex(_Complex float cf) {
  static _Complex float scf;
  auto &[sre, sim] = scf;
  // ok, this is references initialized by constant expressions all the way down
  static_assert(&sre == &__real scf);
  static_assert(&sim == &__imag scf);

  auto [re, im] = cf;
  return re*re + im*im;
}

```
We should visit `DecompositionDecl` and create a local variable but not
a create a dummy value directly.

---------

Signed-off-by: yronglin <[email protected]>

Test Plan: 

Reviewers: 

Subscribers: 

Tasks: 

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Differential Revision: https://phabricator.intern.facebook.com/D60250516

Author: yronglin

clang/lib/AST/Interp/Compiler.cpp

@@ -5239,6 +5239,10 @@ bool Compiler<Emitter>::visitDeclRef(const ValueDecl *D, const Expr *E) {
           return false;
         };
 
+        // DecompositionDecls are just proxies for us.
+        if (isa<DecompositionDecl>(VD))
+          return revisit(VD);
+
         // Visit local const variables like normal.
         if ((VD->hasGlobalStorage() || VD->isLocalVarDecl() ||
              VD->isStaticDataMember()) &&

clang/test/SemaCXX/cxx1z-decomposition.cpp

@@ -1,5 +1,6 @@
 // RUN: %clang_cc1 -std=c++17 -Wc++20-extensions -verify=expected %s
 // RUN: %clang_cc1 -std=c++20 -Wpre-c++20-compat -verify=expected %s
+// RUN: %clang_cc1 -std=c++20 -Wpre-c++20-compat -fexperimental-new-constant-interpreter -verify=expected %s
 
 void use_from_own_init() {
   auto [a] = a; // expected-error {{binding 'a' cannot appear in the initializer of its own decomposition declaration}}