[clang][cas] Add missing test for vfsoverlay + cas-fs caching

We forgot to add a test when this case was fixed.

(cherry picked from commit 22f238e)

Author: benlangmuir

clang/test/ClangScanDeps/modules-cas-fs-vfsoverlay.c

@@ -0,0 +1,51 @@
+// Check cas-fs-based caching works with vfsoverlay files.
+
+// RUN: rm -rf %t
+// RUN: split-file %s %t
+// RUN: sed -e "s|DIR|%/t|g" %t/cdb.json.template > %t/cdb.json
+// RUN: sed -e "s|DIR|%/t|g" %t/vfs.yaml.template > %t/vfs.yaml
+
+// RUN: clang-scan-deps -compilation-database %t/cdb.json -j 1 \
+// RUN:   -format experimental-full -mode preprocess-dependency-directives \
+// RUN:   -cas-path %t/cas -action-cache-path %t/cache > %t/deps.json
+
+// RUN: %deps-to-rsp %t/deps.json --module-name=A > %t/A.rsp
+// RUN: %deps-to-rsp %t/deps.json --tu-index 0 > %t/tu.rsp
+// RUN: %clang @%t/A.rsp
+// RUN: %clang @%t/tu.rsp
+
+//--- cdb.json.template
+[{
+  "directory": "DIR",
+  "command": "clang -fsyntax-only DIR/tu.c -fmodules -fmodules-cache-path=DIR/module-cache -fimplicit-modules -fimplicit-module-maps -ivfsoverlay DIR/vfs.yaml",
+  "file": "DIR/tu.c"
+}]
+
+//--- vfs.yaml.template
+{
+  "version": 0,
+  "case-sensitive": "false",
+  "roots": [
+    {
+      "name": "DIR/A",
+      "type": "directory"
+      "contents": [
+        {
+          "external-contents": "DIR/elsewhere1/A.modulemap",
+          "name": "module.modulemap",
+          "type": "file"
+        }
+      ]
+    }
+  ]
+}
+
+//--- elsewhere1/A.modulemap
+module A { header "A.h" }
+
+//--- A/A.h
+typedef int A_t;
+
+//--- tu.c
+#include "A/A.h"
+A_t a = 0;