This type represents the expression+type arguments you can get in a class or interface
heritage clause section. For class-implements clauses, or interface-extends clauses,
these expressions can only be identifiers or dotted names. For class extends clauses,
these could be any expressions in the future. However, for now, we only support identifiers
and dotted names.
This was we don't get noisy baselines just because a different phase of the compiler reported
the diagnostic.
This helps with Yui's refactoring work to move grammar checks into the type checker.