std::aligned_storage is deprecated in C++23

[StephanTLavavej]

I work on MSVC's C++ Standard Library, where we've recently implemented a C++23 paper that affects your project. (We regularly build many open-source projects to prevent compiler/library regressions, and this also allows us to notify projects of breaking changes that will affect them.)

The C++23 paper is https://www.open-std.org/jtc1/sc22/wg21/docs/papers/2021/p1413r3.pdf "Deprecate std::aligned_storage and std::aligned_union". We've merged a PR (microsoft/STL#2583 ) that makes aligned_storage, aligned_storage_t, aligned_union, and aligned_union_t emit deprecation warnings when compiled in C++23 mode (/std:c++latest). This will be available in VS 2022 17.3 Preview 3. Although your project might not use /std:c++latest right now, we run such test coverage to identify issues that you might encounter in the future.

I searched and one line of affected code appears to be (there may be more):

leveldb/util/no_destructor.h

Lines 40 to 41 in d019e36

	typename std::aligned_storage<sizeof(InstanceType),
	alignof(InstanceType)>::type instance_storage_;

(This emitted a warning when used by another project in our test harness.)

In general, the recommended replacement is to use the alignas keyword. We do provide an "escape hatch" macro to silence the deprecation warning, if necessary.

[marcushultman]

This PR has been open for a while, but it doesn't look like anything prevents merging: #896
cc PR author: @frederick-vs-ja