Workaround a VC++ bug. Microsoft puts the hash_map class in namespace stdext, but

Workaround a VC++ bug.  Microsoft puts the hash_map class in namespace stdext, but
this classes uses a utility function in namespace std.  But Microsoft apparently
assumes everyone will "using namespace std;".  As LLVM doesn't....  Add a
"use std::_Distance;" to get it working.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@19411 91177308-0d34-0410-b5e6-96231b3b80d8