[clang] [CIR] Build out AST consumer patterns to reach the entry point into CIRGen (PR #91007)

[Bruno Cardoso Lopes via cfe-commits]

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@@ -42,6 +47,14 @@ CreateFrontendBaseAction(CompilerInstance &CI) {
   StringRef Action("unknown");
   (void)Action;
 
+  auto UseCIR = CI.getFrontendOpts().UseClangIRPipeline;
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bcardosolopes wrote:

Should this be the case for actual lib/CIR/CodeGen too? MLIR uses `auto` extensively (perhaps because value semantics is common land there) and we have been doing that as well for CIRGen and passes - it's going to be a massive change for us, so we it'd be good to know if that should apply everywhere or only into the areas of Clang isolated from MLIR.

https://github.com/llvm/llvm-project/pull/91007