[libcxx] adds an include-what-you-use (IWYU) mapping file

This makes it possible for programmers to run IWYU and get more accurate
standard library inclusions. Prior to this commit, the following program
would be transformed thusly:

```cpp
// Before
 #include <algorithm>
 #include <vector>

void f() {
  auto v = std::vector{0, 1};
  std::find(std::ranges::begin(v), std::ranges::end(v), 0);
}
```

```cpp
// After
 #include <__algorithm/find.h>
 #include <__ranges/access.h>
 #include <vector>
...
```

There are two ways to fix this issue: to use [comment pragmas](https://github.com/include-what-you-use/include-what-you-use/blob/master/docs/IWYUPragmas.md)
on every private include, or to write a canonical [mapping file](https://github.com/include-what-you-use/include-what-you-use/blob/master/docs/IWYUMappings.md)
that provides the tool with a manual on how libc++ is laid out. Due to
the complexity of libc++, this commit opts for the latter, to maximise
correctness and minimise developer burden.

To mimimise...