natvis: Fix std::unordered_[set|multiset|map|multimap] and add basic_string synthetic to dump nul-delimited substrings.

The std::unordered* type recently broke, with the debugger reporting:
`Unable to find type '__table::__node_pointer' for cast.`

I was not able to find a way to restore the functionality while keeping
the unified definition; instead this change splits the unordered_*set
and unordered_*map definitions which also means it is now reasonable to
use LinkedListItems instead.

This change also adds support for dumping basic_strings with embedded
NUL characters (including highlighting when the string contains them).
An example of where this is particularly useful is when dumping
HttpResponseHeaders's raw_headers_ member.

Change-Id: Ic4101c65bc22bb0227d447ea36546f2fa2481dfb
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/7001755
Reviewed-by: Nico Weber <thakis@chromium.org>
Commit-Queue: Nico Weber <thakis@chromium.org>
Auto-Submit: Erik Anderson <Erik.Anderson@microsoft.com>
Cr-Commit-Position: refs/heads/main@{#1525807}