Fix for #18433 (internal error during xml comment processing)

[Martin521]

Description

Fixes #18433

Remarks

Initially, I wanted to replace the internal error by a warning.
However, the internal error only occurs

• in situations like the one described in Internal error when compiling contents with #line and triple-slash /// comments #18433, where you are not interested in the xml comment, or
• if you have multiple line directives that point to the same line and are followed by xml comments.

So, I settled for just ignoring the repeated xml comments for the same line.
Note that an xml comment will be lost only if it is (according to the line directives) on the same line as another one AND it is different from the other one. Which really does not make sense for any of the scenarios that line directives are meant for.
(Also, implemention-wise, getting to the range for a warning would have been quite complicated.)

Checklist

• Test cases added
• Release notes entry updated

[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