[clang compat] Remove unused llvm::Optional

Clang 125f4457a54a550846732763ee36b1447ec8d66e changed.
The LLVM project is migrating from llvm::Optional to std::optional.

We would previously see llvm::Optional via an indirect include:
    iwyu_preprocessor.h
      iwyu_output.h
        clang/AST/Decl.h
          clang/AST/APValue.h
            clang/Basic/LLVM.h

Since we don't use it, remove the using-declaration.

Signed-off-by: Petr Bred <bredpetr@gmail.com>

iwyu_preprocessor.cc

@@ -44,7 +44,6 @@ using clang::SourceLocation;
 using clang::SourceRange;
 using clang::Token;
 using llvm::errs;
-using llvm::Optional;
 using llvm::StringRef;
 using std::make_pair;
 using std::string;