[clang][Interp] Handle record initialization via CastExpr

[tbaeder]

Summary

In particular, via a CXXFunctionalCastExpr.

clang/lib/AST/Interp/ByteCodeExprGen.cpp

if (const auto CtorExpr = dyn_cast<CXXConstructExpr>(Initializer)) {
     // RVO functions expect a pointer to initialize on the stack.
     // Dup our existing pointer so it has its own copy to use.
     if (!this->emitDupPtr(Initializer))
       return false;
 
     return this->VisitCallExpr(CE);
   } else if (const auto *DIE = dyn_cast<CXXDefaultInitExpr>(Initializer)) {
     return this->visitInitializer(DIE->getExpr());
+  } else if (const auto *CE = dyn_cast<CastExpr>(Initializer)) {
+    return this->visitInitializer(CE->getSubExpr());
   }
 
   return false;
 }
 
 template <class Emitter>
 bool ByteCodeExprGen<Emitter>::visitInitializer(const Expr *Initializer) {
   QualType InitializerType = Initializer->getType();

clang/test/AST/Interp/records.cpp

 struct Ints2 {
   int a = 10;
   int b;
 };
 constexpr Ints2 ints22; // expected-error {{without a user-provided default constructor}} \
                         // expected-error {{must be initialized by a constant expression}} \
                         // ref-error {{without a user-provided default constructor}}
 
+constexpr Ints2 I2 = Ints2{12, 25};
+static_assert(I2.a == 12, "");
+static_assert(I2.b == 25, "");
+
 class C {
   public:
     int a;
     int b;
 
   constexpr C() : a(100), b(200) {}
 };