[P0857R0 Part-B] Allows `require' clauses appearing in

template-template parameters. Although it effects whether a template can be
used as an argument for another template, the constraint seems not to
be checked, nor other major implementations (GCC, MSVC, et al.) check it.

Additionally, Part-A of the document seems to have been implemented.
So mark P0857R0 as completed.

Differential Revision: https://reviews.llvm.org/D134128