[clang][test] Avoid some C++14 warnings in discrim-union.cpp (#146829)

Before this patch, we emitted a bunch of irrelevant (to this test)
warnings:
```
  ../clang/test/SemaCXX/discrim-union.cpp:49:24: warning: 'constexpr' non-static member function will not be implicitly 'const' in C++14; add 'const' to avoid a change in behavior [-Wconstexpr-not-const]
   49 |     constexpr const T &get(select<0>) { return val; }
      |                        ^
      |                                       const
../clang/test/SemaCXX/discrim-union.cpp:50:104: warning: 'constexpr' non-static member function will not be implicitly 'const' in C++14; add 'const' to avoid a change in behavior [-Wconstexpr-not-const]
   50 |     template<unsigned N> constexpr const decltype(static_cast<const rest_t&>(rest).get(select<N-1>{})) get(select<N>) {
      |                                                                                                        ^
      |                                                                                                                       const
../clang/test/SemaCXX/discrim-union.cpp:82:22: warning: 'constexpr' non-static member function will not be implicitly 'const' in C++14; add 'const' to avoid a change in behavior [-Wconstexpr-not-const]
   82 |   constexpr unsigned index() noexcept { return elem; }
      |                      ^
      |                              const
../clang/test/SemaCXX/discrim-union.cpp:88:33: warning: 'constexpr' non-static member function will not be implicitly 'const' in C++14; add 'const' to avoid a change in behavior [-Wconstexpr-not-const]
   88 |   constexpr const_get_result<N> get() {
      |                                 ^
      |                                       const
../clang/test/SemaCXX/discrim-union.cpp:96:22: warning: 'constexpr' non-static member function will not be implicitly 'const' in C++14; add 'const' to avoid a change in behavior [-Wconstexpr-not-const]
   96 |   constexpr const U &get() {
      |                      ^
      |                            const
5 warnings generated.
```

Author: tbaederr

clang/test/SemaCXX/discrim-union.cpp

@@ -1,4 +1,6 @@
-// RUN: %clang_cc1 -std=c++11 %s -fsyntax-only -fcxx-exceptions
+// RUN: %clang_cc1 -std=c++11 %s -fsyntax-only -fcxx-exceptions -verify
+
+// expected-no-diagnostics
 
 template<typename T> struct remove_reference { typedef T type; };
 template<typename T> struct remove_reference<T&> { typedef T type; };
@@ -46,8 +48,8 @@ namespace detail {
         val.~T();
     }
 
-    constexpr const T &get(select<0>) { return val; }
-    template<unsigned N> constexpr const decltype(static_cast<const rest_t&>(rest).get(select<N-1>{})) get(select<N>) {
+    constexpr const T &get(select<0>) const { return val; }
+    template<unsigned N> constexpr const decltype(static_cast<const rest_t&>(rest).get(select<N-1>{})) get(select<N>) const {
       return rest.get(select<N-1>{});
     }
   };
@@ -79,21 +81,21 @@ class either {
   // FIXME: declare a destructor iff any element has a nontrivial destructor
   //~either() { impl.destroy(elem); }
 
-  constexpr unsigned index() noexcept { return elem; }
+  constexpr unsigned index() const noexcept { return elem; }
 
   template<unsigned N> using const_get_result =
     decltype(static_cast<const impl_t&>(impl).get(detail::select<N>{}));
 
   template<unsigned N>
-  constexpr const_get_result<N> get() {
+  constexpr const_get_result<N> get() const {
     // Can't just use throw here, since that makes the conditional a prvalue,
     // which means we return a reference to a temporary.
     return (elem != N ? throw_<const_get_result<N>>("bad_either_get")
                       : impl.get(detail::select<N>{}));
   }
 
   template<typename U>
-  constexpr const U &get() {
+  constexpr const U &get() const {
     return get<impl_t::index(detail::type<U>())>();
   }
 };