clang: Restore support for clang-cl on non-Windows hosts

The frontend variant detection logic added by commit 53fbe23 (clang:
introduce CMAKE_<lang>_COMPILER_FRONTEND_VARIANT, 2019-02-20,
v3.15.0-rc1~41^2~8) assumes that `clang-cl` only runs on a Windows host.
It is also available on non-Windows hosts.  Fix the condition.

Fixes: #19544

Author: bradking

Modules/CMakeDetermineCompilerId.cmake

@@ -138,7 +138,7 @@ function(CMAKE_DETERMINE_COMPILER_ID lang flagvar src)
 
   set(_variant "")
   if("x${CMAKE_${lang}_COMPILER_ID}" STREQUAL "xClang")
-    if(CMAKE_HOST_WIN32 AND "x${CMAKE_${lang}_SIMULATE_ID}" STREQUAL "xMSVC")
+    if("x${CMAKE_${lang}_SIMULATE_ID}" STREQUAL "xMSVC")
       if(CMAKE_GENERATOR MATCHES "Visual Studio")
         set(CMAKE_${lang}_COMPILER_FRONTEND_VARIANT "MSVC")
       else()