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