`Union[('A', 'B')]` resolves strings as `Literal["A"]` instead of the actual type — causing false E1002/E1053 errors

[ThiruvarankanM]

Description

When a type alias is defined using Union[] with string names (forward references), the type checker misreads those strings as plain text values instead of class names. This floods the output with false type errors.

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Minimal Reproducible Example

import from typing { Union }

glob MyType = Union[('Foo', 'Bar')];

obj Foo {}
obj Bar {}

def make -> MyType {
    return Foo();  # Should be fine — Foo is a valid member of MyType
}

Expected: No errors. Foo is a valid member of MyType.

Actual:

error[E1002]: Cannot return Foo, expected Literal["Foo"] | Literal["Bar"]

---

Root Cause

The type checker evaluates each string inside Union[(...)] using get_type_of_expression. When called on a plain string like 'Foo' outside a type-annotation position, it returns Literal["Foo"] (the text) — not the Foo class.

So Union[('Foo', 'Bar')] becomes Literal["Foo"] | Literal["Bar"] instead of Foo | Bar.

The function _try_resolve_forward_ref already exists to resolve string names to real types — it is used correctly for has x: 'Foo' and -> 'Foo' annotations — but it was never called inside the Union[] handler.

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Real-World Impact

This pattern is used in doc_ir.jac:

glob DocType = Union[
    ('Doc', 'Text', 'Line', 'Group', 'Indent', 'Concat', 'IfBreak', 'Align')
];

Because DocType resolves to string literals instead of actual classes, every method in doc_ir_gen_pass.impl.jac that uses DocType as a return type or parameter type gets flagged:

error[E1002]: Cannot return Concat, expected Literal["Doc"] | Literal["Text"] | ...
error[E1053]: Cannot assign Text to parameter of type Literal["Doc"] | Literal["Text"] | ...

~1,440 false errors in a single file — all from one misread.

[ThiruvarankanM]

#6110