Add MSVC configuration with /Zc:__cppstd set to c++20

Demonstrate bug #343 - since MSVC offers individual options to control
/std:c++20, /Zc:__cplusplus, and /Zc:preprocessor, it's possible to have
a combination where the compiler is in C++20 mode (and says so), but
the traditional preprocessor (which not support __VA_OPT__) is in use.

This configuration is commonly trigged by Qt6, which requires c++17,
suggests c++20, sets /Zc:__cpluspls (so it can #ifdef based on compiler),
but doesn't care about the preprocessor. MSVC still doesn't enable the
new /Zc:preprocessor by default, even with /std:c++20 or /permissive-.

Author: puetzk

.github/workflows/ci.yml

@@ -1675,6 +1675,13 @@ jobs:
             cc: "cl", cxx: "cl",
             cxx_standard: 20,
           }
+          - {
+            name: "Windows MSVC 2022 C++20 __cppstd",
+            os: windows-2022,
+            cc: "cl", cxx: "cl",
+            cxx_standard: 20,
+            cxx_flags: "/Zc:__cplusplus"
+          }
           - {
             name: "Windows MSVC 2022 C++20 preprocessor",
             os: windows-2022,
@@ -1729,6 +1736,13 @@ jobs:
             cc: "cl", cxx: "cl",
             cxx_standard: 20,
           }
+          - {
+            name: "Windows MSVC 2019 C++20 __cppstd",
+            os: windows-2019,
+            cc: "cl", cxx: "cl",
+            cxx_standard: 20,
+            cxx_flags: "/Zc:__cplusplus"
+          }
           - {
             name: "Windows MSVC 2019 C++20 preprocessor",
             os: windows-2019,