Change directory_entry::path() to return a const std::string & instead of

Change directory_entry::path() to return a const std::string & instead of
a StringRef, for the benefit of clients that want the result as a
nul-terminated string. Clients that expect a StringRef will get one via
the implicit conversion.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@126784 91177308-0d34-0410-b5e6-96231b3b80d8