[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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@@ -7,6 +7,10 @@ set(LLVM_LINK_COMPONENTS
 
 get_property(dialect_libs GLOBAL PROPERTY MLIR_DIALECT_LIBS)
 
+# CIRGenAction reaches into the Transforms-private LowerModule helper to
+# stand up the AST-free LoweringPrepare pipeline.
+include_directories(${CMAKE_CURRENT_SOURCE_DIR}/../Dialect/Transforms)
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bcardosolopes wrote:

This is the part I'd push back on hardest. Two libraries now `-I` into another library's source dir to get at a private header, and the comment here says so out loud (same thing in `Lowering/CMakeLists.txt`).

If `LowerModule` is what CIRGen and the lowering pipeline agree on, it's an interface, so put it in `clang/include/clang/CIR/` and depend on it properly. Reaching in with `include_directories` gets you a dependency that doesn't show up in the build graph and that nobody will notice breaking.

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