Poison memory in variant destroy

In the version of `__destroy` that does not destruct any part of
`__data`, MSAN is unable to see that `__data` is logically
uninitialized. This allows false negatives such as the following to
run without any MSAN diagnostic.

```
std::variant<double, int> v;
v.emplace<double>();
double& d = std::get<double>(v);
v.emplace<int>();
if (d) ...
```

With these changes, MSAN reports uninitialized memory:

```
==32202==WARNING: MemorySanitizer: use-of-uninitialized-value
    #0 0x5557b64820aa in main /home/ccotter/variant_msan.cpp:19:9
    #1 0x7fbfacd88554 in __libc_start_main /usr/src/debug/glibc-2.17-c758a686/csu/../csu/libc-start.c:266
    #2 0x5557b63ed40d in _start (/home/ccotter/a.out+0x3140d)
```

Author: ccotter

libcxx/include/variant

@@ -261,6 +261,10 @@ namespace std {
 #include <new>
 #include <version>
 
+#if __has_feature(memory_sanitizer)
+#include <sanitizer/msan_interface.h>
+#endif
+
 // standard-mandated includes
 
 // [variant.syn]
@@ -781,6 +785,9 @@ _LIBCPP_VARIANT_DESTRUCTOR(
     _Trait::_TriviallyAvailable,
     _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR_SINCE_CXX20 ~__dtor() = default,
     _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR_SINCE_CXX20 void __destroy() noexcept {
+#if __has_feature(memory_sanitizer)
+      __sanitizer_dtor_callback(&this->__data, sizeof(this->__data));
+#endif
       this->__index = __variant_npos<__index_t>;
     } _LIBCPP_EAT_SEMICOLON);