[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