Fix ci_emscripten: make sweep_allocator::construct() not viable for const types (#6036)

* Tests: make sweep_allocator::construct() not viable for const types

The newest libc++ (emsdk latest, used by ci_emscripten) probes
construct(const T*, const T&) in an unevaluated context while checking
whether copies are trivial. For the test allocator, instantiating the
noexcept specification then fails to compile, because static_cast<void*>
cannot drop the const qualifier. Constrain construct() to non-const types,
which makes the probe fail in the SFINAE context instead of with a hard
error.

Co-Authored-By: Claude Sonnet 5.5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01N5NasEsrzHhSNouxSoEwqz
Signed-off-by: Niels Lohmann <mail@nlohmann.me>

* Fix CI: astyle and clang-tidy for sweep_allocator::construct()

astyle 3.4.13 reformats the enable_if<!...> default template argument
(Check amalgamation). Delete the const overload instead, which makes the
libc++ probe for const objects fail in the SFINAE context just the same
and needs no <type_traits>.

Co-Authored-By: Claude Sonnet 5.5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01N5NasEsrzHhSNouxSoEwqz
Signed-off-by: Niels Lohmann <mail@nlohmann.me>

---------

Signed-off-by: Niels Lohmann <mail@nlohmann.me>
Co-authored-by: Claude Sonnet 5.5 <noreply@anthropic.com>

tests/src/unit-allocator.cpp

@@ -1079,6 +1079,11 @@ struct sweep_allocator
         ::new (static_cast<void*>(p)) U(std::forward<Args>(args)...);
     }
 
+    // libc++ probes whether construct() is usable for const objects (when
+    // checking for trivial copies); this must not be viable then
+    template<class U, class... Args>
+    void construct(const U* /*unused*/, const Args& ... /*unused*/) = delete;
+
     template<class U>
     void destroy(U* p) noexcept
     {