[clang] [Serialization] Fix swapped PPOpts/ExistingPPOpts parameters. NFC.

The two first parameters of checkPreprocessorOptions are "PPOpts, ExistingPPOpts".
All other callers of the function pass them consistently.

This avoids confusion when working on the code.

Differential Revision: https://reviews.llvm.org/D129277

Author: mstorsjo

clang/lib/Serialization/ASTReader.cpp

@@ -5171,8 +5171,9 @@ namespace {
     bool ReadPreprocessorOptions(const PreprocessorOptions &PPOpts,
                                  bool Complain,
                                  std::string &SuggestedPredefines) override {
-      return checkPreprocessorOptions(ExistingPPOpts, PPOpts, nullptr, FileMgr,
-                                      SuggestedPredefines, ExistingLangOpts);
+      return checkPreprocessorOptions(PPOpts, ExistingPPOpts, /*Diags=*/nullptr,
+                                      FileMgr, SuggestedPredefines,
+                                      ExistingLangOpts);
     }
   };