[clang-tidy] Fix a compiler warning.
Rename the Preprocessor field to fix the
declaration of ‘std::unique_ptr<clang::Preprocessor> clang::tooling::ExpandModularHeadersPPCallbacks::Preprocessor’ changes the meaning of ‘Preprocessor’ from ‘class clang::Preprocessor’ [-fpermissive]
warning.
llvm-svn: 356756