[clang] [clang] Extract in-memory module cache writes from `ASTWriter` (PR #190062)

[Cyndy Ishida via cfe-commits]

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@@ -1488,6 +1488,18 @@ static bool compileModuleImpl(CompilerInstance &ImportingInstance,
     ImportingInstance.getModuleCache().updateModuleTimestamp(ModuleFileName);
   }
 
+  // This isn't strictly necessary, but it's more efficient to extract the AST
+  // file (which may be wrapped in an object file) now rather than doing so
+  // repeatedly in the readers.
+  const PCHContainerReader &Rdr = ImportingInstance.getPCHContainerReader();
+  StringRef ExtractedBuffer = Rdr.ExtractPCH(*Buffer);
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cyndyishida wrote:

A patch to find/replace `PCHContainerReader` with e.g. `ASTFileContainerReader` and `ExtractPCH` to `ExtractASTFile`. It's yet another place where the code reads in a different way than what it is actually doing. For example, it took me a little longer to reason about the relationship of the `ModuleFileName` and `Buffer` in this diff because it wasn't obvious that `ExtractPCH` isn't strictly for PCHs. The interaction between `ExtractPCH` & `addBuiltPCM` also takes some more work to reason about. 

https://github.com/llvm/llvm-project/pull/190062