[driver] Add regression test

Author: RichBarton-Arm

clang/test/Driver/flang/classic-flang-emit-flang-llvm.f95

@@ -0,0 +1,10 @@
+! REQUIRES: classic_flang
+
+! Check that the -emit-flang-llvm option dumps LLVM IR pre-optimisation
+
+! RUN: %clang --driver-mode=flang -emit-flang-llvm -S -o %t.ll %s -### 2>&1 \
+! RUN:   | FileCheck %s
+! CHECK-NOT: argument unused during compilation: '-S'
+! CHECK: "{{.*}}flang1"
+! CHECK-NEXT: "{{.*}}flang2"
+! CHECK-NOT: "{{.*}}clang{{.*}}" "-cc1"