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

[Ben Langmuir via cfe-commits]

================
@@ -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;
----------------
benlangmuir wrote:

What does `OnCI` mean? Is there a clearer name we could choose and/or could we document the role of this callback?

Also, how do we know function_ref is safe here?

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