[libc++][NFC] Comment cleanup for <type_traits> (#130422)

- Aligns all version comments to line 65.
- Consistently uses `// since C++` instead of `// C++`, in lowercase as
used in most version comments.
- Consistently uses `class` to introduce type template parameters.
- Consistently uses `inline constexpr` for variable templates.
- Corrects the comment for `bool_constant` to `// since C++17` as it's a
C++17 feature.
- Changes the class-key of `result_of` to `struct`, which follows N4659
[depr.meta.types] and the actual usage in libc++.
- Adds missed `// since C++17` for `is_(nothrow_)invocable(_r)`.
- Moves the comments for `is_nothrow_convertible_v` to the part for
variable templates.
- Removes duplicated comments for `true_type` and `false_type`.

Author: frederick-vs-ja