clang-cl: Restore rules to create preprocessed and assembly output

In commit d993ebd4ca (clang-cl: Add '--' before source file, 2020-07-28)
we accidentally replaced these rules with those to compile an object
file.

Modules/Platform/Windows-MSVC.cmake

@@ -338,8 +338,8 @@ macro(__windows_compiler_msvc lang)
     # paths starting with /U as macro undefines, so we need to put a -- before the
     # input file path to force it to be treated as a path.
     string(REPLACE "-c <SOURCE>" "-c -- <SOURCE>" CMAKE_${lang}_COMPILE_OBJECT "${CMAKE_${lang}_COMPILE_OBJECT}")
-    string(REPLACE "-c <SOURCE>" "-c -- <SOURCE>" CMAKE_${lang}_CREATE_PREPROCESSED_SOURCE "${CMAKE_${lang}_COMPILE_OBJECT}")
-    string(REPLACE "-c <SOURCE>" "-c -- <SOURCE>" CMAKE_${lang}_CREATE_ASSEMBLY_SOURCE "${CMAKE_${lang}_COMPILE_OBJECT}")
+    string(REPLACE "-c <SOURCE>" "-c -- <SOURCE>" CMAKE_${lang}_CREATE_PREPROCESSED_SOURCE "${CMAKE_${lang}_CREATE_PREPROCESSED_SOURCE}")
+    string(REPLACE "-c <SOURCE>" "-c -- <SOURCE>" CMAKE_${lang}_CREATE_ASSEMBLY_SOURCE "${CMAKE_${lang}_CREATE_ASSEMBLY_SOURCE}")
 
   elseif(MSVC_VERSION GREATER_EQUAL 1913)
     # At least MSVC toolet 14.13 from VS 2017 15.6