[VirtualFileSystem] Add unit test that showcases another YAMLVFSWriter bug

This scenario generates another broken YAML mapping as illustrated below.

  {
    'type': 'directory',
    'name': "c",
    'contents': [
      ,
      {
        'type': 'directory',
        'name': "d",
        'contents': [
          ,
          {
            'type': 'directory',
            'name': "e",
            'contents': [
              {
                'type': 'file',
                'name': "f",
                'external-contents': "//root/a/c/d/e/f"
              }                    {
                'type': 'file',
                'name': "g",
                'external-contents': "//root/a/c/d/e/g"
              }
            ]
          }
        ]
      }
    ]
  },

Author: JDevlieghere

llvm/unittests/Support/VirtualFileSystemTest.cpp

@@ -2271,6 +2271,40 @@ TEST_F(VFSFromYAMLTest, YAMLVFSWriterTest2) {
   EXPECT_FALSE(FS.get() != nullptr);
 }
 
+TEST_F(VFSFromYAMLTest, YAMLVFSWriterTest3) {
+  ScopedDir TestDirectory("virtual-file-system-test", /*Unique*/ true);
+  ScopedDir _a(TestDirectory + "/a");
+  ScopedFile _ab(TestDirectory + "/a/b", "");
+  ScopedDir _ac(TestDirectory + "/a/c");
+  ScopedDir _acd(TestDirectory + "/a/c/d");
+  ScopedDir _acde(TestDirectory + "/a/c/d/e");
+  ScopedFile _acdef(TestDirectory + "/a/c/d/e/f", "");
+  ScopedFile _acdeg(TestDirectory + "/a/c/d/e/g", "");
+  ScopedDir _ah(TestDirectory + "/a/h");
+  ScopedFile _ahi(TestDirectory + "/a/h/i", "");
+
+  vfs::YAMLVFSWriter VFSWriter;
+  VFSWriter.addDirectoryMapping(_a.Path, "//root/a");
+  VFSWriter.addFileMapping(_ab.Path, "//root/a/b");
+  VFSWriter.addDirectoryMapping(_ac.Path, "//root/a/c");
+  VFSWriter.addDirectoryMapping(_acd.Path, "//root/a/c/d");
+  VFSWriter.addDirectoryMapping(_acde.Path, "//root/a/c/d/e");
+  VFSWriter.addFileMapping(_acdef.Path, "//root/a/c/d/e/f");
+  VFSWriter.addFileMapping(_acdeg.Path, "//root/a/c/d/e/g");
+  VFSWriter.addDirectoryMapping(_ahi.Path, "//root/a/h");
+  VFSWriter.addFileMapping(_ahi.Path, "//root/a/h/i");
+
+  std::string Buffer;
+  raw_string_ostream OS(Buffer);
+  VFSWriter.write(OS);
+  OS.flush();
+
+  IntrusiveRefCntPtr<ErrorDummyFileSystem> Lower(new ErrorDummyFileSystem());
+  IntrusiveRefCntPtr<vfs::FileSystem> FS = getFromYAMLRawString(Buffer, Lower);
+  // FIXME: Spurious comma separator before first file entry in directory.
+  EXPECT_FALSE(FS.get() != nullptr);
+}
+
 TEST_F(VFSFromYAMLTest, YAMLVFSWriterTestHandleDirs) {
   ScopedDir TestDirectory("virtual-file-system-test", /*Unique*/ true);
   ScopedDir _a(TestDirectory + "/a");