[lldb] Ignore linkage diagnostic for LLDBSwigPythonBreakpointCallbackFunction (NFC)

Ignore `-Wreturn-type-c-linkage` diagnostics for `LLDBSwigPythonBreakpointCallbackFunction`.

The function is defined in `python-wrapper.swig` which uses `extern "C" { ... }` blocks.
The declaration of this function in `ScriptInterpreterPython.cpp` already uses these
same pragmas to silence the warning there.

This prevents `-Werror` builds from failing.

Differential Revision: https://reviews.llvm.org/D98368