Fix calls to functions with const T&& parameters. (#7208)

This also fixes passing a value expression to a forwarding reference,
since we currently deduce a `const T&&` parameter in that case.

We were accidentally looking at the type of the thunk parameter (which
is never an rvalue reference) rather than the type of the callee
parameter.

toolchain/check/cpp/thunk.cpp

@@ -497,7 +497,7 @@ static auto BuildThunkParamRef(
   // Cast to an xvalue when using pass-by-`var` or when initializing an rvalue
   // reference (which might be passed by value if it's const-qualified).
   if (passing_mode == SemIR::ClangDeclSignature::PassingMode::ByVar ||
-      thunk_param->getType()->isRValueReferenceType()) {
+      (!type.isNull() && type->isRValueReferenceType())) {
     call_arg = clang::ImplicitCastExpr::Create(
         sema.getASTContext(), call_arg->getType(), clang::CK_NoOp, call_arg,
         nullptr, clang::ExprValueKind::VK_XValue, clang::FPOptionsOverride());