[lldb] Emit type annotation in SWIG generated Python code.

[JDevlieghere]

Summary

The Python code generated by SWIG is compatible with both Python 2 and Python 3. The -py3 option enables Python 2 incompatible features such as function annotations and abstract base classes.

Given that this only affects the generated code (and as such doesn't cause merge conflicts downstream) we're happy to enable it, i.e. this doesn't conflict with our ask in https://lists.llvm.org/pipermail/lldb-dev/2020-August/016388.html.

[kastiglione]

This will be nice!

[labath]

cool

[lanza]

Thanks, Jonas! This generates bindings but they aren't entirely functional as it generates type annotations like "lldb::SBDebugger" which Python LSPs can't figure out. I ended up adding -py3 and then running some sed post-processing to convert "lldb::\(.*\)" to \1 etc: https://github.com/lanza/lldbpybind. That lldb.py works pretty well with pyright getting type info. Figuring out how to teach the build system + swig to generate proper type annotations might require a similar post processing step.

lldb/bindings/python/CMakeLists.txt

 add_custom_command(
   OUTPUT ${CMAKE_CURRENT_BINARY_DIR}/LLDBWrapPython.cpp
   OUTPUT ${CMAKE_CURRENT_BINARY_DIR}/lldb.py
   DEPENDS ${SWIG_SOURCES}
   DEPENDS ${SWIG_INTERFACES}
   DEPENDS ${SWIG_HEADERS}
   COMMAND ${SWIG_EXECUTABLE}
       ${SWIG_COMMON_FLAGS}
       -I${CMAKE_CURRENT_SOURCE_DIR}
       -c++
       -shadow
       -python
+      -py3
       -threads
       -outdir ${CMAKE_CURRENT_BINARY_DIR}
       -o ${CMAKE_CURRENT_BINARY_DIR}/LLDBWrapPython.cpp
       ${CMAKE_CURRENT_SOURCE_DIR}/python.swig
   VERBATIM
   COMMENT "Building LLDB Python wrapper")
 
 add_custom_target(swig_wrapper_python ALL DEPENDS