isDeclarationSpecifiers did not handle some cases of placeholder-type-specifiers with
type-constraints, causing parsing bugs in abbreviated constructor templates.
Add comprehensive handling of type-constraints to isDeclarationSpecifier.
The code for parsing of type-constraints in compound-requirements was not adapted for the new TryAnnotateTypeConstraint which
caused compound-requirements with scope specifiers to ignore them.
Also add regression tests for scope specifiers in type-constraints in more contexts.