Preserve error elaboration for indexed access type assignments

[WhiteMinds]

Fixes #63206

When assigning to an indexed access type (e.g. this["faa"]), the checker resolves the constraint and performs a structural comparison that generates specific error messages (missing properties, type mismatches, etc.). However, reportRelationError unconditionally clears errorInfo before reporting the generic "could be instantiated with an arbitrary type" message, discarding the useful elaboration.

This change preserves errorInfo when the target is an IndexedAccess type, so users see both the generic message and the specific details.

Before

this.foo = {}
// Type '{}' is not assignable to type 'this["faa"]'.
//   'this["faa"]' could be instantiated with an arbitrary type which could be unrelated to '{}'.

After

this.foo = {}
// Type '{}' is not assignable to type 'this["faa"]'.
//   'this["faa"]' could be instantiated with an arbitrary type which could be unrelated to '{}'.
//     Property 'key' is missing in type '{}' but required in type '{ key: number | null; }'.

this.foo = {key: "str"}
// Type '{ key: string; }' is not assignable to type 'this["faa"]'.
//   'this["faa"]' could be instantiated with an arbitrary type which could be unrelated to '{ key: string; }'.
//     Type '{ key: string; }' is not assignable to type '{ key: number | null; }'.
//       Types of property 'key' are incompatible.
//         Type 'string' is not assignable to type 'number'.

Two existing baselines updated to reflect the additional elaboration (no errors removed, only more specific detail added).

[WhiteMinds]

@microsoft-github-policy-service agree