[lldb] On Windows, silence warning with latest MSVC

This fixes:
```
[6083/7449] Building CXX object tools\lldb\source\Commands\CMakeFiles\lldbCommands.dir\CommandObjectFrame.cpp.obj
C:\git\llvm-project\lldb\source\Commands\CommandObjectFrame.cpp(497) : warning C4715: 'CommandObjectFrameVariable::ScopeRequested': not all control paths return a value
```