automoc: Remove existing output file before invoking moc

Remove the output file before invoking moc in case the case of source file has
changed on disk. Recent versions of clang warn when the case of the include
directive (which does change) doesn't match the case of the file on disk.

Source/cmQtAutoMocUic.cxx

@@ -1981,6 +1981,9 @@ void cmQtAutoMocUicT::JobCompileMocT::Process()
   std::string const& sourceFile = this->Mapping->SourceFile->FileName;
   std::string const& outputFile = this->Mapping->OutputFile;
 
+  // Remove output file in case the case of the source file has changed
+  cmSystemTools::RemoveFile(outputFile);
+
   // Compose moc command
   std::vector<std::string> cmd;
   {