Add test case to exercise IRgen "unsupported" path (mostly cleanup).

llvm-svn: 58286

clang/test/CodeGen/unsupported.c

@@ -0,0 +1,6 @@
+// RUN: clang -verify -emit-llvm -o %t %s 
+
+int f0(int x) {
+  int vla[x]; // expected-error {{cannot codegen this variable-length array yet}}
+  return vla[x-1];
+}