[lldb/SWIG] Refactor extensions to be non Python-specific

The current SWIG extensions for the string conversion operator is Python
specific because it uses the PythonObjects. This means that the code
cannot be reused for other SWIG supported languages such as Lua.

This reimplements the extensions in a more generic way that can be
reused.

Differential revision: https://reviews.llvm.org/D72377