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

cmake/modules/ErrorFlags.cmake

@@ -65,6 +65,10 @@ elseif(MSVC)
 			"/WX" # Treat warnings as errors
 		)
 	endif()
+
+	# Silence deprecation warnings for the std::atomic_...<std::shared_ptr> family of functions.
+	# TODO: Remove once C++20's std::atomic<std::shared_ptr> is fully supported.
+	add_compile_definitions("_SILENCE_CXX20_OLD_SHARED_PTR_ATOMIC_SUPPORT_DEPRECATION_WARNING")
 endif()
 
 # Add the flags to the whole directory tree. We use the third-party flags for