[clang-cl] Set _MSVC_LANG to 202002L with /std:c++20

[thakis]

Summary

MSVC says this should be 202002L for /std:c++20, and of VS16.11
that's indeed the case (older versions warn that they don't
understand /std:c++20, and then cl.exe defaults to C++14 and
sets _MSVC_LANG to 201402 accordingly).

[thakis]

For reference: https://godbolt.org/z/3rnKovjTG

[hans]

lgtm

clang/lib/Basic/Targets/OSTargets.cpp

if (Opts.MSCompatibilityVersion) {
     // FIXME We cannot encode the revision information into 32-bits
     Builder.defineMacro("_MSC_BUILD", Twine(1));
 
     if (Opts.CPlusPlus11 && Opts.isCompatibleWithMSVC(LangOptions::MSVC2015))
       Builder.defineMacro("_HAS_CHAR16_T_LANGUAGE_SUPPORT", Twine(1));
 
     if (Opts.isCompatibleWithMSVC(LangOptions::MSVC2015)) {
       if (Opts.CPlusPlus20)
-        Builder.defineMacro("_MSVC_LANG", "201705L");
+        Builder.defineMacro("_MSVC_LANG", "202002L");
       else if (Opts.CPlusPlus17)
         Builder.defineMacro("_MSVC_LANG", "201703L");
       else if (Opts.CPlusPlus14)
         Builder.defineMacro("_MSVC_LANG", "201402L");
     }
   }
 
   if (Opts.MicrosoftExt) {

clang/test/Preprocessor/predefined-win-macros.c

 // CHECK-MS: #define __GCC_HAVE_SYNC_COMPARE_AND_SWAP_1 1
 // CHECK-MS: #define __GCC_HAVE_SYNC_COMPARE_AND_SWAP_2 1
 // CHECK-MS: #define __GCC_HAVE_SYNC_COMPARE_AND_SWAP_4 1
 // CHECK-MS: #define __GCC_HAVE_SYNC_COMPARE_AND_SWAP_8 1
 // CHECK-MS-NOT: GNU
 // CHECK-MS-NOT: GXX
 
 // RUN: %clang_cc1 %s -x c++ -E -dM -triple i686-pc-win32 -fms-extensions -fms-compatibility \
-// RUN:     -fms-compatibility-version=19.00 -std=c++2a -o - | FileCheck -match-full-lines %s --check-prefix=CHECK-MS-CPP2A
+// RUN:     -fms-compatibility-version=19.00 -std=c++20 -o - | FileCheck -match-full-lines %s --check-prefix=CHECK-MS-CPP2A
 // CHECK-MS-CPP2A: #define _MSC_VER 1900
-// CHECK-MS-CPP2A: #define _MSVC_LANG 201705L
+// CHECK-MS-CPP2A: #define _MSVC_LANG 202002L
 
 // RUN: %clang_cc1 -triple i386-windows %s -E -dM -o - \
 // RUN:   | FileCheck -match-full-lines %s --check-prefix=CHECK-X86-WIN
 
 // CHECK-X86-WIN-NOT: #define WIN32 1
 // CHECK-X86-WIN-NOT: #define WIN64 1
 // CHECK-X86-WIN-NOT: #define WINNT 1
 // CHECK-X86-WIN: #define _WIN32 1