[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.
```