[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'
```

Author: aganea

lldb/source/Plugins/Process/Utility/NativeRegisterContextDBReg.cpp

@@ -283,7 +283,7 @@ bool NativeRegisterContextDBReg::ClearHardwareWatchpoint(uint32_t wp_index) {
     LLDB_LOG_ERROR(
         log, std::move(error),
         "unable to set watchpoint: failed to read debug registers: {0}");
-    return LLDB_INVALID_INDEX32;
+    return false;
   }
 
   if (wp_index >= m_max_hwp_supported)