libstdc++: Fix std::allocator_traits::construct constraints [PR108619]

Using std::is_constructible in the constraints introduces a spurious
dependency on the type being destructible, which should not be required
for constructing with an allocator. The test case shows a case where the
type has a private destructor, which can be destroyed by the allocator,
but std::is_destructible and std::is_constructible are false.

Similarly, using is_nothrow_constructible in the noexcept-specifiers
for the construct members of allocator_traits and std::allocator,
__gnu_cxx::__new_allocator, and __gnu_cxx::__malloc_allocator gives the
wrong answer if the type isn't destructible.
We need a new type trait to define those correctly, so that we only
check if the placement new-expression is nothrow after using
is_constructible to check that it would be well-formed.

Instead of just fixing the overly restrictive constraint to check for
placement new, rewrite allocator_traits in terms of 'if constexpr' using
variable templates and the detection idiom.

Although we can use 'if constexpr' and variable templates in C++11 with
appropriate uses of diagnostic pragmas, we can't have constexpr
functions with multiple return statements. This means that in C++11 mode
the _S_nothrow_construct and _S_nothrow_destroy helpers used for
noexcept-specifiers still need to be overlaods using enable_if. Nearly
everything else can be simplified to reduce overload resolution and
enable_if checks.

libstdc++-v3/ChangeLog:

	PR libstdc++/108619
	* include/bits/alloc_traits.h (__allocator_traits_base): Add
	variable templates for detecting which allocator operations are
	supported.
	(allocator_traits): Use 'if constexpr' instead of dispatching to
	overloads constrained with enable_if.
	(allocator_traits<allocator<T>>::construct): Use Construct if
	construct_at is not supported. Use
	__is_nothrow_new_constructible for noexcept-specifier.
	(allocator_traits<allocator<void>>::construct): Use
	__is_nothrow_new_constructible for noexcept-specifier.
	* include/bits/new_allocator.h (construct): Likewise.
	* include/ext/malloc_allocator.h (construct): Likewise.
	* include/std/type_traits (__is_nothrow_new_constructible): New
	variable template.
	* testsuite/20_util/allocator/89510.cc: Adjust expected results.
	* testsuite/ext/malloc_allocator/89510.cc: Likewise.
	* testsuite/ext/new_allocator/89510.cc: Likewise.
	* testsuite/20_util/allocator_traits/members/108619.cc: New test.

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