Upgrade to C++20 (#7554)

* Compile in C++20 mode

* Fix implicit lambda captures of `this` by `[=]`

Those implicit captures were deprecated in C++20

* Silence MSVC atomic std::shared_ptr warning

Unfortunately std::atomic<std::shared_ptr> (P0718R2) is not supported by
GCC until GCC 12 and still is not supported by Clang or Apple Clang, so
it looks like we will continue using std::atomic_load for the time being

* Use C++20 in RemoteVstPlugin

* Simplification

* Add comment

* Fix bitwise operations between different enumeration types

* Revert "Fix bitwise operations between different enumeration types"

This reverts commit d45792cd72.

* Use a helper function to combine keys and modifiers

* Remove AnalyzeTemporaryDtors from .clang-tidy

AnalyzeTemporaryDtors was deprecated in clang-tidy 16 and fully removed
in clang-tidy 18

* Use C++20 in .clang-format

* Use bitwise OR

Prevents issues if any enum flags in `args` have bits in common

include/DeprecationHelper.h

@@ -27,7 +27,10 @@
 #ifndef LMMS_DEPRECATIONHELPER_H
 #define LMMS_DEPRECATIONHELPER_H
 
+#include <type_traits>
+
 #include <QFontMetrics>
+#include <QKeySequence>
 #include <QWheelEvent>
 
 namespace lmms
@@ -64,6 +67,30 @@ inline QPoint position(QWheelEvent *wheelEvent)
 #endif
 }
 
+
+namespace detail
+{
+
+template<typename T>
+inline constexpr bool IsKeyOrModifier = std::is_same_v<T, Qt::Key>
+	|| std::is_same_v<T, Qt::Modifier> || std::is_same_v<T, Qt::KeyboardModifier>;
+
+} // namespace detail
+
+
+/**
+ * @brief Combines Qt key and modifier arguments together,
+ * replacing `A | B` which was deprecated in C++20
+ * due to the enums being different types. (P1120R0)
+ * @param args Any number of Qt::Key, Qt::Modifier, or Qt::KeyboardModifier
+ * @return The combination of the given keys/modifiers as an int
+ */
+template<typename... Args, std::enable_if_t<(detail::IsKeyOrModifier<Args> && ...), bool> = true>
+constexpr int combine(Args... args)
+{
+	return (0 | ... | static_cast<int>(args));
+}
+
 } // namespace lmms
 
 #endif // LMMS_DEPRECATIONHELPER_H