Field-backed properties: support field references in speculative SemanticModel

[cston]

Support field references in speculative SemanticModel for property accessors, including for auto-properties and partial properties.

This may require public API support to indicate that a statement or expression should be parsed in the context of an accessor, so that field is treated as keyword.

See related tests PropertyFieldKeywordTests.SpeculativeSemanticModel_* in earlier feature branch features/semi-auto-props.

See also #75904 (comment).

[CyrusNajmabadi]

Note: i would not expect field in a spec semantic model to bind to anything, if the initial semantic model didn't have field in the prop. But i also would expect it to not crash.