Fix lifetime. StringRef doesn't allocate storage.
Implement Jira #ROOT-5273 ACLiC through #include

Implement support of #include "myFile.C+" in Cling

- Add a new class TPPClingCallbacks in TClingCallbacks.cxx
- Add two new functions TCling__CompileMacro and TCling__SplitAclicMode TCling.cxx, both used by TPPClingCallbacks

The FileNotFound() method of TPPClingCallbacks is called via Callbacks->FileNotFound(Filename, RecoveryPath) in Preprocessor::HandleIncludeDirective(), allowing to compile code via ACLiC when specifying #include "myfile.C+", and hence suppressing preprocessor error message like:
input_line_23:1:10: fatal error: 'myfile.C+' file not found