[VirtualFileSystem] Make gcc<7.5 happy after 75d71800

There is a libstdc++ stl_map.h bug that is only back ported to 7.5.
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=78595#c13

```
/tmp/opt/gcc-7.3.0/include/c++/7.3.0/bits/stl_tree.h:2091:28: error: no matching function for call to ‘std::_Rb_tree<std::__cxx11::basic_string<char>, std::pair<const std::__cxx11::basic_string<char>, std::unique_ptr<llvm::vfs::detail::InMemoryNode> >, std::_Select1st<std::pair<const std::__cxx11::basic_string<char>, std::unique_ptr<llvm::vfs::detail::InMemoryNode> > >, std::less<std::__cxx11::basic_string<char> >, std::allocator<std::pair<const std::__cxx11::basic_string<char>, std::unique_ptr<llvm::vfs::detail::InMemoryNode> > > >::_M_get_insert_unique_pos(std::pair<llvm::StringRef, std::unique_ptr<llvm::vfs::detail::InMemoryNode> >::first_type&)’
  = _M_get_insert_unique_pos(_KeyOfValue()(__v));
    ~~~~~~~~~~~~~~~~~~~~~~~~^~~~~~~~~~~~~~~~~~~~
```

Just construct a std::string from StringRef to work around it.