Code completion: fix getting qualifier expression in do statements in type decls

[auduchinok]

Fixes various cases of broken code completion inside do statements inside type declarations:

type T() =
    do
        "".ToString().L{caret}

Example 1:

Example 2:

This PR also simplifies the code completion testing helpers and updates tests added in #18519, so that PR should be merged first.

[github-actions]

❗ Release notes required

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✅ Found changes and release notes in following paths:

Change path	Release notes path	Description
src/Compiler	docs/release-notes/.FSharp.Compiler.Service/9.0.300.md