Try to fix windows build more after r198136.

The command that cmd.exe is complaining about is:
cmd.exe /c cd /D C:\bb-win7\cmake-clang-i686-mingw32\build\tools\lto && cmake -E echo EXPORTS > symbol.def && type C:/bb-win7/cmake-clang-i686-mingw32/llvm-project/llvm/tools/lto/lto.exports >> symbol.def

Maybe quoting the filename helps.

llvm-svn: 198140

llvm/cmake/modules/AddLLVM.cmake

@@ -37,7 +37,7 @@ function(add_llvm_symbol_exports target_name export_file)
 
     add_custom_command(OUTPUT symbol.def
       COMMAND ${CMAKE_COMMAND} -E echo "EXPORTS" > symbol.def
-      COMMAND ${CAT} ${export_file} >> symbol.def
+      COMMAND ${CAT} "${export_file}" >> symbol.def
       DEPENDS ${export_file}
       VERBATIM
       COMMENT "Creating export file for ${target_name}")