[clang] [clang] Move state out of `PreprocessorOptions` (1/n) (PR #86358)

[Ben Langmuir via cfe-commits]

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@@ -34,12 +34,17 @@ class InitOnlyAction : public FrontendAction {
 
 /// Preprocessor-based frontend action that also loads PCH files.
 class ReadPCHAndPreprocessAction : public FrontendAction {
+  llvm::function_ref<void(CompilerInstance &)> OnCI;
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benlangmuir wrote:

My concern is that nothing prevents using this frontend action from some other code where it wouldn't be safe.  Maybe we can make it a unique_function?

https://github.com/llvm/llvm-project/pull/86358