[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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+// RUN: %clang_cc1 -triple x86_64-unknown-linux-gnu -fclangir -emit-cir %s
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bcardosolopes wrote:

We don't have any IR to test just yet. @lanza you could use `FileCheck` with `--allow-empty` here for now. You could also rename this test `emit-cir.c` since the purpose for now is to not crash (as it would before this PR).

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