[clang] [CIR] Run the CIR lowering pipeline without a live ASTContext and accept serialized .cir as cc1 input (PR #219048)

[Bruno Cardoso Lopes via cfe-commits]

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@@ -293,10 +290,19 @@ struct LoweringPreparePass
   }
 
   ///
-  /// AST related
-  /// -----------
+  /// Target / language fact source
+  /// -----------------------------
 
-  clang::ASTContext *astCtx;
+  /// Target/LangOpts/CXXABI bundle the pass reads instead of going to a live
+  /// ASTContext.  Set from the surrounding cc1 invocation by
+  /// runCIRToCIRPasses (or by the .cir input path in CIRGenAction).  Always
+  /// non-null while the pass runs.
+  cir::LowerModule *lowerModule = nullptr;
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bcardosolopes wrote:

"Always non-null while the pass runs" isn't enforced anywhere, and `createLoweringPreparePass()` with no arguments is still declared, which hands you exactly this null. Every use below is an unguarded deref, so that's a segfault rather than a diagnostic.

You already did the right thing a few hundred lines down, swapping the `assert` on a missing `static_local_info` for an `emitError` and a bail. Same treatment here.

https://github.com/llvm/llvm-project/pull/219048