Thread-Safty: Add Mutex per Interpreter

    Have a mutex for each interpreter instance create.
    Only use the lock at writes to the AST.

Use hashmap to identify Sema & AST from individual Decl

    We need to identify which interpreter a Decl belongs to, when using multiple interpreter.
    We do it by checking which `clang::ASTContext` the `clang::Decl` belongs
    We maintain a map: `clang::ASTContext -> Cpp::InterpreterInfo`. Using this map, we identify the correct interpreter.

    Note that the mutex is stored in the InterpreterInfo class.

    There are 2 usecases for this:
    1. We can now lock the correct interpreter making it thread safe.
    2. User of `libCppInterOp` need not set the correct active interpreter using `Cpp::ActivateInterpreter`,
       this information can be retrived using the map.

Author: Vipul-Cariappa