`std::vector<int&>` generates nearly 170 lines of diagnostic, most of
which are redundant, and it's only helpful if you are already familiar
with the rule that containers can't hold reference types. This commit
reduces it to only a handful of lines, all of which are dedicated to
clearly communicating the problem. It also applies this to all the
other containers, and for all non-cv-qualified and non-object types.

These static_asserts are placed at the very top of each container
because they short-circuit further instantiation errors, thereby
leading a smaller set of diagnostics for the programmer. Placing them
in `std::allocator` (which is the common denominator for all containers)
doesn't do the short circuiting, and we thus end up with several
unhelpful diagnostics after the helpful one.

This commit only cleans up things that are already diagnosed as
compile-time errors. In particular, `map<int&&, int>` should be
ill-formed, per [associative.reqmts.general]/p8. libc++ and libstdc++
currently permit that, but not `map<int&, int>`. As such, this
commit only adds a static assert for `map<K&, V>`, and defers diagnosing
valid productions that should be ill-formed to a future commit.

I think the best way to enact `git clang-format` might be to use `git
clang-format <hash-before-your-commit-first>` instead of `git
clang-format HEAD~n`. This ensures that all commits in a PR are
formatted, instead of just the last n (which one might miscount, like I
probably did).

The trick is to run

```
$ git-clang-format --diff <base-hash> --extensions ,cpp,h | patch -p1
```

A Clang pragma that lets us "hide" macro expansion from diagnostics so
that users don't need to read implementation details would be a good
thing to explore in the future.

Co-authored-by: Louis Dionne <ldionne.2@gmail.com>

`std::llocator` apparently supports unbounded arrays prior to C++20, so
we need to change the `is_array` check to only check for bounded arrays.

Nikolas pointed out that checking all the traits in all cases will
probably grind builds to a halt as we improve our diagnostics, so this
commit makes a point of reducing the number of instantiations as much as
possible, and preserves the highly specific messages.

This reverts commit bb82c95.

Co-authored-by: Nikolas Klauser <nikolasklauser@berlin.de>

Co-authored-by: Nikolas Klauser <nikolasklauser@berlin.de>