[CIR] Change unreachable to diagnostic for ill-equipped clang

clang/include/clang/Basic/DiagnosticFrontendKinds.td

@@ -132,6 +132,8 @@ def err_fe_no_pch_in_dir : Error<
     "no suitable precompiled header file found in directory '%0'">;
 def err_fe_action_not_available : Error<
     "action %0 not compiled in">;
+def err_fe_cir_not_built : Error<"clang IR support not available, rebuild "
+                                 "clang with -DCLANG_ENABLE_CIR=ON">;
 def err_fe_invalid_multiple_actions : Error<
     "'%0' action ignored; '%1' action specified previously">;
 def err_fe_invalid_alignment : Error<

clang/lib/FrontendTool/ExecuteCompilerInvocation.cpp

@@ -81,7 +81,8 @@ CreateFrontendBaseAction(CompilerInstance &CI) {
 #if CLANG_ENABLE_CIR
     return std::make_unique<cir::EmitCIRAction>();
 #else
-    llvm_unreachable("CIR suppport not built into clang");
+    CI.getDiagnostics().Report(diag::err_fe_cir_not_built);
+    return nullptr;
 #endif
   case EmitHTML:               return std::make_unique<HTMLPrintAction>();
   case EmitLLVM: {

clang/test/Frontend/cir-not-built.c

@@ -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
+// UNSUPPORTED: cir-support
+
+// RUN: not %clang_cc1 -emit-cir %s 2>&1 | FileCheck %s
+// CHECK: error: clang IR support not available, rebuild clang with -DCLANG_ENABLE_CIR=ON
+
+int main(void) {
+  return 0;
+}

clang/test/lit.cfg.py

@@ -224,6 +224,10 @@ def have_host_clang_repl_cuda():
         )
     )
 
+# ClangIR support
+if config.clang_enable_cir:
+    config.available_features.add("cir-support")
+
 llvm_config.add_tool_substitutions(tools, tool_dirs)
 
 config.substitutions.append(