[lldb] Debugger: reuse ExecutionContextRef to create ExecutionContext from Target

The Debugger didn't take the Process's run lock, that causes deadlocks and races
after applying https://reviews.llvm.org/D92164 revision. Since ExecutionContextRef
does the same job correctly, Debugger::GetSelectedExecutionContext just can use it
to build execution context upon the selected target.

GitOrigin-RevId: a39bcbca92e169baeb8b2c55dff90141ddd53888
1 file changed