Support: Move OnDiskHashTable from clang to llvm

This introduces clang's Basic/OnDiskHashTable.h into llvm as
Support/OnDiskHashTable.h. I've taken the opportunity to add doxygen
comments and run the file through clang-format, but other than the
namespace changing from clang:: to llvm:: the API is identical.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@206438 91177308-0d34-0410-b5e6-96231b3b80d8