Explicitly require C++20 (facebook#9575)

Summary:
HHVM (and some dependencies) now make heavy use of C++20 features such as coroutines, so make this requirement explicit in the root listfile. This also allows conveniently setting/forwarding the standard level in subprojects by referencing CMAKE_CXX_STANDARD.

Pull Request resolved: facebook#9575

Reviewed By: Wilfred

Differential Revision: D67959847

fbshipit-source-id: 539c201f9108bed07844ef9e9ca1f205261e4446

Author: mszabo-wikia

CMakeLists.txt

@@ -5,6 +5,9 @@ cmake_policy(SET CMP0045 NEW)
 # ditto for add_dependencies(): https://cmake.org/cmake/help/v3.0/policy/CMP0046.html
 cmake_policy(SET CMP0046 NEW)
 
+set(CMAKE_CXX_STANDARD 20)
+set(CMAKE_CXX_STANDARD_REQUIRED ON)
+
 # This needs to be done before any languages are enabled or
 # projects are created.
 INCLUDE("${CMAKE_CURRENT_SOURCE_DIR}/CMake/VisualStudioToolset.cmake")