[Tests] Enable C++ interop for Runtime in the overlay verification test.

The Runtime module requires C++ interop enabled, because that's
the only way to robustly fix things so that it is able to declare various
types and constants without clashing with system headers.

To make this work, we also need to add a missing invocation to sil-opt to
make it pick the correct C++ runtime library.

rdar://143050566

Author: al45tair

lib/DriverTool/sil_opt_main.cpp

@@ -794,6 +794,7 @@ int sil_opt_main(ArrayRef<const char *> argv, void *MainAddr) {
   }
 
   Invocation.getLangOptions().EnableCXXInterop = options.EnableCxxInterop;
+  Invocation.computeCXXStdlibOptions();
 
   Invocation.getLangOptions().UnavailableDeclOptimizationMode =
       options.UnavailableDeclOptimization;

validation-test/SIL/verify_all_overlays.py

@@ -38,11 +38,13 @@
     # TODO: fix the DifferentiationUnittest module.
     if module_name == "DifferentiationUnittest":
         continue
-    # Backtracing needs its own additional modules in the module path
+    # Runtime needs its own additional modules in the module path, and
+    # also needs C++ interop enabled
     if module_name == "Runtime":
         extra_args = ["-I", os.path.join(source_dir, "stdlib",
                                          "public", "RuntimeModule", "modules"),
-                      "-I", os.path.join(source_dir, "include")]
+                      "-I", os.path.join(source_dir, "include"),
+                      "--enable-experimental-cxx-interop"]
     # _Concurrency needs its own additional modules in the module path
     if module_name == "_Concurrency":
         extra_args = ["-I", os.path.join(source_dir, "stdlib",