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

[Erich Keane via cfe-commits]

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+// RUN: %clang_cc1 -triple x86_64-unknown-linux-gnu -fclangir -emit-cir %s
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erichkeane 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).

Both of those are OK enough ideas to me.  So at this point, we're just emitting a blank-file, right?   Perhaps   `--allow-empty` plus a `// CHECK-NOT: *` or something.

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