[clang] [CIR] Change unreachable to diagnostic for ill-equipped clang (PR #152614)

[Arseniy Zaostrovnykh via cfe-commits]

================
@@ -0,0 +1,12 @@
+// Test that using -emit-cir when clang is not built with CIR support gives a proper error message
+// instead of crashing.
+
+// This test should only run when CIR support is NOT enabled
+// REQUIRES: !cir-support
----------------
necto wrote:

Have you considered
```suggestion
// UNSUPPORTED: cir-support
```
?

https://github.com/llvm/llvm-project/pull/152614