[rootcling] Remove llvm::cl dependency by using ROOT's optparse

This would potentially remove issues like

```
CommandLine Error: Option 'W' registered more than once!
```

caused by `llvm::cl` registering the same options multiple times
when ROOT is linked against a shared `libLLVM`.

Author: devajithvs

core/dictgen/CMakeLists.txt

@@ -51,6 +51,7 @@ target_include_directories(Dictgen PRIVATE
   ${CMAKE_SOURCE_DIR}/core/meta/inc
   ${CMAKE_SOURCE_DIR}/core/foundation/inc
   ${CMAKE_BINARY_DIR}/ginclude
+  ${CMAKE_SOURCE_DIR}/main/src
   ${CLING_INCLUDE_DIRS}
 )