[lldb] Silence warning with latest MSVC

Fixes several of these:
```
[3370/3822] Building CXX object tools\lldb\source\Plugins\Process\U...lldbPluginProcessUtility.dir\NativeRegisterContextDBReg_x86.cpp.ob
C:\git\llvm-project\lldb\source\Plugins\Process\Utility\NativeRegisterContextDBReg_x86.h(23): warning C4589: Constructor of abstract class 'lldb_private::NativeRegisterContextDBReg_x86' ignores initializer for virtual base class 'lldb_private::NativeRegisterContextRegisterInfo'
C:\git\llvm-project\lldb\source\Plugins\Process\Utility\NativeRegisterContextDBReg_x86.h(23): note: virtual base classes are only initialized by the most-derived type
```