[libcxx] Disable long double -> int128 test with msan

[vitalybuka]

Summary

On x86 long double is 80 bit with padding, which produces
uninitialized bits in the result.
This will trigger errors with -fsanitize-memory-param-retval.

Can be triggered with D123979.

[vitalybuka]

rebase

[vitalybuka]

rebase

[vitalybuka]

We can't land D123980 without this one

[vitalybuka]

ping

[vitalybuka]

ping

libcxx/test/std/numerics/bit/bit.cast/bit_cast.pass.cpp

 #include <array>
 #include <bit>
 #include <cassert>
 #include <cmath>
 #include <cstdint>
 #include <cstring>
 #include <limits>
 
+#include "test_macros.h"
+
 // std::bit_cast does not preserve padding bits, so if T has padding bits,
 // the results might not memcmp cleanly.
 template<bool HasUniqueObjectRepresentations = true, typename T>
 void test_roundtrip_through_buffer(T from) {
     struct Buffer { char buffer[sizeof(T)]; };
     Buffer middle = std::bit_cast<Buffer>(from);
     T to = std::bit_cast<T>(middle);
     Buffer middle2 = std::bit_cast<Buffer>(to);
for (long double i : {0.0l, 1.0l, -1.0l, 10.0l, -10.0l, 1e10l, 1e-10l, 1e100l, 1e-100l,
                           std::numeric_limits<long double>::quiet_NaN()}) {
         // Note that x86's `long double` has 80 value bits and 48 padding bits.
         test_roundtrip_through_nested_T<false>(i);
         test_roundtrip_through_buffer<false>(i);
 
 #if __SIZEOF_LONG_DOUBLE__ == __SIZEOF_DOUBLE__
         test_roundtrip_through<double, false>(i);
 #endif
-#if defined(__SIZEOF_INT128__) && __SIZEOF_LONG_DOUBLE__ == __SIZEOF_INT128__
+#if defined(__SIZEOF_INT128__) && __SIZEOF_LONG_DOUBLE__ == __SIZEOF_INT128__ &&                                       \
+    !TEST_HAS_FEATURE(memory_sanitizer) // Some bits are just padding.
         test_roundtrip_through<__int128_t, false>(i);
         test_roundtrip_through<__uint128_t, false>(i);
 #endif
     }
 
     return true;
 }