Drive-by changes in <type_traits>

- aligning all version comments to line 65,
- using `// since C++` instead of `// C++`, in lowercase as used in most version comments,
- changing the comments for `bool_constant` to `// since C++17` as it's a C++17 feature,
- changing the class-key of `result_of` to `struct`, which follows N4659 [depr.meta.types] and the actual usage in libc++,
- consistently using `class` to introduce type template parameters,
- consistently using `inline constexpr` for variable templates,
- moving the comments for `is_nothrow_convertible_v` to the part for variable templates, and
- removing duplicated comments for `true_type` and `false_type`.

Author: frederick-vs-ja