Revert "[VFS] Add support for handling path traversals"

This reverts commit r261551 due to failing tests in windows bots:

http://lab.llvm.org:8011/builders/clang-x64-ninja-win7/builds/10054

Failing Tests (4):
    Clang-Unit :: Basic/BasicTests.exe/VFSFromYAMLTest.CaseInsensitive
    Clang-Unit :: Basic/BasicTests.exe/VFSFromYAMLTest.DirectoryIteration
    Clang-Unit :: Basic/BasicTests.exe/VFSFromYAMLTest.MappedFiles
    Clang-Unit :: Basic/BasicTests.exe/VFSFromYAMLTest.UseExternalName

llvm-svn: 261654