[lldb] Use current execution context in SBDebugger

Use `GetSelectedExecutionContext()` instead of
`GetCommandInterpreter().GetExecutionContext()` in
`SBDebugger::GetInternalVariableValue/SBDebugger::SetInternalVariable`. The
execution context in the command interpreter might be empty, if no commands has
been executed yet (it is updated only when handling commands or completions --
e.g.
https://github.com/llvm/llvm-project/blob/main/lldb/source/Interpreter/CommandInterpreter.cpp#L1855).

Reviewed By: teemperor

Differential Revision: https://reviews.llvm.org/D95761