Fixes #16367 During semantic analysis, we try to parse all strings as types, including those inside Literal[]. Previously, we preserved the original string in the `UnboundType.original_str_expr` attribute, but if a type is parsed as a Union, we didn't have a place to put the value. This PR instead always wraps string types in a RawExpressionType node, which now optionally includes a `.node` attribute containing the parsed type. This way, we don't need to worry about preserving the original string as a custom attribute on different kinds of types that can appear in this context. The downside is that more code needs to be aware of RawExpressionType.

Uh oh!