Avoid nullptr dereferencing of 'Constraint'; NFC

Klocwork static code analysis exposed this bug:
Pointer 'Constraint' returned from call to function
'cast_or_null<clang::ConceptSpecializationExpr,clang::Expr>' may
be NULL and will be dereferenced in the statement following it

Replace 'cast_or_null' with 'cast' so that the latter can assert
when it encounters a NULL.

This also removes an 'assert' that is covered by the previous
call to 'cast<>'.