[Windows on Arm] Don't use an env var for the msvc compatibility version

[DavidSpickett]

Summary

Long story short the cmake command isn't run in a shell but ninja
generated commands are. This is why %MSVC_VER% wasn't expanded but
%BUILTINS% wasn't used at configure time so it didn't matter there.

I looked at setting LLVM_TEMPORARILY_ALLOW_OLD_TOOLCHAIN but
both the minimum and soft error versions are set to 19.27
so it doesn't help us.

Instead just hardcode the expected verison. We'll probably always
be behind as we're using a released clang-cl to build latest
clang-cl but at least it won't change that often.

[rovka]

LGTM.

[DavidSpickett]

Pushed as https://github.com/llvm/llvm-zorg/commit/6220393b1a7718f2122d545a988116f499ad18ae.

buildbot/osuosl/master/config/builders.py

'factory' : ClangBuilder.getClangCMakeBuildFactory(
                     checkout_flang=True,
                     # Only enabling checks we expect to work.
                     checks=['check-flang'],
                     extra_cmake_args=[
                         "-DCMAKE_TRY_COMPILE_CONFIGURATION=Release",
                         "-DLLVM_DEFAULT_TARGET_TRIPLE=aarch64-pc-windows-msvc",
                         "-DLLVM_HOST_TRIPLE=aarch64-pc-windows-msvc",
                         "-DLLVM_TARGET_ARCH=AArch64",
-                        # The MSVC_VER environment variable is expected to already exist
-                        # on the worker when it is launched.
-                        "-DCMAKE_C_FLAGS=-fms-compatibility-version=%MSVC_VER%",
-                        "-DCMAKE_CXX_FLAGS=-fms-compatibility-version=%MSVC_VER%",
+                        "-DCMAKE_C_FLAGS=-fms-compatibility-version=19.27",
+                        "-DCMAKE_CXX_FLAGS=-fms-compatibility-version=19.27",
                         # The BUILTINS environment variable is expected to already exist
                         # on the worker when it is launched.
                         "-DCMAKE_EXE_LINKER_FLAGS=%BUILTINS%",
                         "-DCMAKE_SHARED_LINKER_FLAGS=%BUILTINS%",
                         "-DCMAKE_STATIC_LINKER_FLAGS=%BUILTINS%",
                         "-DCMAKE_MODULE_LINKER_FLAGS=%BUILTINS%",
                         # FIXME: compiler-rt\lib\sanitizer_common\sanitizer_unwind_win.cpp assumes WIN64 is x86_64,
                         #        so, before that's fixed, disable everything that triggers its build.
'factory' : ClangBuilder.getClangCMakeBuildFactory(
                     checks=[], # Disable testing until MCJIT failures are fixed
                     useTwoStage=True,
                     testStage1=False,
                     extra_cmake_args=[
                         "-DCMAKE_TRY_COMPILE_CONFIGURATION=Release",
                         "-DLLVM_DEFAULT_TARGET_TRIPLE=aarch64-pc-windows-msvc",
                         "-DLLVM_HOST_TRIPLE=aarch64-pc-windows-msvc",
                         "-DLLVM_TARGET_ARCH=AArch64",
-                        # The MSVC_VER environment variable is expected to already exist
-                        # on the worker when it is launched.
-                        "-DCMAKE_C_FLAGS=-fms-compatibility-version=%MSVC_VER%",
-                        "-DCMAKE_CXX_FLAGS=-fms-compatibility-version=%MSVC_VER%",
+                        "-DCMAKE_C_FLAGS=-fms-compatibility-version=19.27",
+                        "-DCMAKE_CXX_FLAGS=-fms-compatibility-version=19.27",
                         # FIXME: compiler-rt\lib\sanitizer_common\sanitizer_unwind_win.cpp assumes WIN64 is x86_64,
                         #        so, before that's fixed, disable everything that triggers its build.
                         "-DCOMPILER_RT_BUILD_SANITIZERS=OFF",
                         "-DCOMPILER_RT_BUILD_MEMPROF=OFF",
                         "-DCOMPILER_RT_BUILD_XRAY=OFF"])},
 
     {'name' : 'clang-x64-windows-msvc',
     'tags'  : ["clang"],