[lldb] Fix erroneous return value
Found when building with MSVC on Windows, was seeing:
```
[2703/7138] Building CXX object tools\lldb\source\Plugins\Process\Utility\CMakeFiles\lldbPluginProcessUtility.dir\NativeRegisterContextDBReg.cpp.obj
C:\git\llvm-project\lldb\source\Plugins\Process\Utility\NativeRegisterContextDBReg.cpp(286): warning C4305: 'return': truncation from 'unsigned int' to 'bool'
```
