[YAMLVFSWriter] Add explicit assert that every file has a parent dir

[jkorous]

Summary

This is a leftover from me poking the code. There's an implicit assumption that every file has a parent - which if violated blows up later in implementation of containedIn(). I don't have any reasonable reproducer except adding root ("/") as file to the mapping but that doesn't make much sense.

Unsure if this should land, happy to hear opinions.

llvm/lib/Support/VirtualFileSystem.cpp

if (IsOverlayRelative.hasValue()) {
     OS << "  'overlay-relative': '" << (UseOverlayRelative ? "true" : "false")
        << "',\n";
   }
   OS << "  'roots': [\n";
 
   if (!Entries.empty()) {
     const YAMLVFSEntry &Entry = Entries.front();
     bool first_entry_is_directory = Entry.IsDirectory;
-    StringRef Dir =
-        first_entry_is_directory ? Entry.VPath : path::parent_path(Entry.VPath);
+    StringRef Dir;
+    if (first_entry_is_directory) {
+      Dir = Entry.VPath;
+    } else {
+      Dir = path::parent_path(Entry.VPath);
+      assert(!Dir.empty() && "No parent directory");
+    }
     startDirectory(Dir);
 
     StringRef RPath = Entry.RPath;
     if (UseOverlayRelative) {
       unsigned OverlayDirLen = OverlayDir.size();
       assert(RPath.substr(0, OverlayDirLen) == OverlayDir &&
              "Overlay dir must be contained in RPath");
       RPath = RPath.slice(OverlayDirLen, RPath.size());