[clang][Interp] Fix three-way comparison detection

Instead of using !T && CPlusPlus, just check the BinaryOperator's
opcode. Turns out we also hit this code path for some assignments
of structs in C++.

clang/lib/AST/Interp/ByteCodeExprGen.cpp

@@ -464,7 +464,7 @@ bool ByteCodeExprGen<Emitter>::VisitBinaryOperator(const BinaryOperator *BO) {
   // Special case for C++'s three-way/spaceship operator <=>, which
   // returns a std::{strong,weak,partial}_ordering (which is a class, so doesn't
   // have a PrimType).
-  if (!T && Ctx.getLangOpts().CPlusPlus) {
+  if (!T && BO->getOpcode() == BO_Cmp) {
     if (DiscardResult)
       return true;
     const ComparisonCategoryInfo *CmpInfo =

clang/test/SemaCXX/conditional-expr.cpp

@@ -1,5 +1,7 @@
 // RUN: %clang_cc1 -fcxx-exceptions -fexceptions -fsyntax-only -verify=expected,expected-cxx11 -std=c++11 -Wsign-conversion %s
+// RUN: %clang_cc1 -fcxx-exceptions -fexceptions -fsyntax-only -verify=expected,expected-cxx11 -std=c++11 -Wsign-conversion %s -fexperimental-new-constant-interpreter
 // RUN: %clang_cc1 -fcxx-exceptions -fexceptions -fsyntax-only -verify=expected,expected-cxx17 -std=c++17 -Wsign-conversion %s
+// RUN: %clang_cc1 -fcxx-exceptions -fexceptions -fsyntax-only -verify=expected,expected-cxx17 -std=c++17 -Wsign-conversion %s -fexperimental-new-constant-interpreter
 
 // C++ rules for ?: are a lot stricter than C rules, and have to take into
 // account more conversion options.