Add a copy of unordered_dense.h from [1]. This file implements an efficient hash map and hash set with a nice C++ interface (a near drop-in for std::unordered_map and std::unordered_set). This is expected to be used to replace uses of `htab_t`, for improved code readability and type safety. Performance-wise, it is preferred to the std types (std::unordered_map and std::unordered_set) due to it using open addressing vs closed addressing for the std types. I chose this particular implementation because it is simple to use, it's a standalone header and it typically performs well in benchmarks I have seen (e.g. [2]). The license being MIT, my understanding is that it's not a problem to use it and re-distribute it. Add two additional files, gdbsupport/unordered_map.h and gdbsupport/unordered_set.h, which make the map and set offered by gdbsupport/unordered_dense.h available as gdb::unordered_map and gdb::unordered_set. [1] https://github.com/martinus/unordered_dense [2] https://jacksonallan.github.io/c_cpp_hash_tables_benchmark/#conclusion-which-hash-table-to-choose Change-Id: I0c7469ccf9a14540465479e58b2a5140a2440a7d Approved-By: Tom Tromey <tom@tromey.com>