Clang/Windows: Clarify name of internal runtime library flags variables

Also avoid extra spaces in resulting flag strings.

Modules/Platform/Windows-Clang.cmake

@@ -89,17 +89,17 @@ macro(__windows_compiler_clang_gnu lang)
     set(CMAKE_${lang}_COMPILE_OPTIONS_MSVC_RUNTIME_LIBRARY_MultiThreadedDebugDLL -D_DEBUG -D_DLL -D_MT -Xclang --dependent-lib=msvcrtd)
 
     if(CMAKE_MSVC_RUNTIME_LIBRARY_DEFAULT)
-      set(__ADDED_FLAGS "")
-      set(__ADDED_FLAGS_DEBUG "")
+      set(_RTL_FLAGS "")
+      set(_RTL_FLAGS_DEBUG "")
     else()
-      set(__ADDED_FLAGS_DEBUG "-D_DEBUG -D_DLL -D_MT -Xclang --dependent-lib=msvcrtd")
-      set(__ADDED_FLAGS "-D_DLL -D_MT -Xclang --dependent-lib=msvcrt")
+      set(_RTL_FLAGS_DEBUG " -D_DEBUG -D_DLL -D_MT -Xclang --dependent-lib=msvcrtd")
+      set(_RTL_FLAGS " -D_DLL -D_MT -Xclang --dependent-lib=msvcrt")
     endif()
 
-    string(APPEND CMAKE_${lang}_FLAGS_DEBUG_INIT " -g -Xclang -gcodeview -O0 ${__ADDED_FLAGS_DEBUG}")
-    string(APPEND CMAKE_${lang}_FLAGS_MINSIZEREL_INIT " -Os -DNDEBUG ${__ADDED_FLAGS}")
-    string(APPEND CMAKE_${lang}_FLAGS_RELEASE_INIT " -O3 -DNDEBUG ${__ADDED_FLAGS}")
-    string(APPEND CMAKE_${lang}_FLAGS_RELWITHDEBINFO_INIT " -O2 -g -DNDEBUG -Xclang -gcodeview ${__ADDED_FLAGS}")
+    string(APPEND CMAKE_${lang}_FLAGS_DEBUG_INIT " -g -Xclang -gcodeview -O0${_RTL_FLAGS_DEBUG}")
+    string(APPEND CMAKE_${lang}_FLAGS_MINSIZEREL_INIT " -Os -DNDEBUG${_RTL_FLAGS}")
+    string(APPEND CMAKE_${lang}_FLAGS_RELEASE_INIT " -O3 -DNDEBUG${_RTL_FLAGS}")
+    string(APPEND CMAKE_${lang}_FLAGS_RELWITHDEBINFO_INIT " -O2 -g -DNDEBUG -Xclang -gcodeview${_RTL_FLAGS}")
   endif()
   set(CMAKE_INCLUDE_SYSTEM_FLAG_${lang} "-isystem ")
   set(CMAKE_${lang}_LINKER_SUPPORTS_PDB ON)
@@ -109,8 +109,8 @@ macro(__windows_compiler_clang_gnu lang)
   set(CMAKE_${lang}_COMPILE_OPTIONS_USE_PCH -Xclang -include-pch -Xclang <PCH_FILE> -Xclang -include -Xclang <PCH_HEADER>)
   set(CMAKE_${lang}_COMPILE_OPTIONS_CREATE_PCH -Xclang -emit-pch -Xclang -include -Xclang <PCH_HEADER> -x ${__pch_header_${lang}})
 
-  unset(__ADDED_FLAGS)
-  unset(__ADDED_FLAGS_DEBUG)
+  unset(_RTL_FLAGS)
+  unset(_RTL_FLAGS_DEBUG)
   string(TOLOWER "${CMAKE_BUILD_TYPE}" BUILD_TYPE_LOWER)
   set(CMAKE_${lang}_STANDARD_LIBRARIES_INIT "-lkernel32 -luser32 -lgdi32 -lwinspool -lshell32 -lole32 -loleaut32 -luuid -lcomdlg32 -ladvapi32 -loldnames")