Fixed the MSVC build when looking for Clang builtin headers in the wrong location.

src/CppParser/premake4.lua

@@ -17,7 +17,7 @@ project "CppSharp.CppParser"
 
   local copy = os.is_windows() and "xcopy /Q /E /Y /I" or "cp -rf";
   local headers = path.getabsolute(path.join(LLVMRootDir, "lib/"))
-  if os.isdir(headers) then
+  if os.isdir(path.join(headers, "clang")) then
     postbuildcommands { copy .. " " .. headers .. " %{cfg.targetdir}" }
   end