Honour -ivfsoverlay in ASTUnit to match clang

This allows code indexing, etc. to use the VFS in the same way as the
compiler.

llvm-svn: 206309
diff --git a/clang/test/Index/Inputs/module_needs_vfs.h b/clang/test/Index/Inputs/module_needs_vfs.h
new file mode 100644
index 0000000..d79cc3f
--- /dev/null
+++ b/clang/test/Index/Inputs/module_needs_vfs.h
@@ -0,0 +1,4 @@
+@import BaseModuleNeedsVFS;
+inline void module_needs_vfs(void) {
+  base_module_needs_vfs();
+}