Previously a C++20 requires-clause was only supported on `function`
declarations. However, the C++ standard allows a require-clause on
class/union templates, alias templates, and variable templates, and
also allows a requires clause after each template parameter list, not
just the final one.
This moves the requiresClause to be a property of `ASTTemplateParams`
rather than `ASTDeclaration` to better match the C++ grammar and
allows requires clauses in many places that are supported by C++20 but
were not previously allowed by Sphinx, namely:
- On class templates, alias templates, and variable templates
- After each template parameter list, not just the last one.
- After the template parameter list in template template parameters.
When encoding the id, the requires clause of the last template
parameter list is treated specially in order to preserve compatibility
with existing v4 ids.