Improve SA1642 and SA1643 code fix

[pdelvo]

Fixes #415.

With these changes the code fix does a better job detecting mistakes in the standard text. If the standard text has the wrong class name in its text it will fix that instead of inserting a new standard text at the beginning of the summary. Examples:

It replaces a text form of the class name with a see tag:

/// <summary>
/// Initializes a new instance of the Foo class.
/// </summary>

->

/// <summary>
/// Initializes a new instance of the <see cref="Foo"/> class.
/// </summary>

It fixes a wrong see tag:

/// <summary>
/// Initializes a new instance of the <see cref="Bar"/> class.
/// </summary>

->

/// <summary>
/// Initializes a new instance of the <see cref="Foo"/> class.
/// </summary>

And it fixes a wrong class text with the correct class name:

/// <summary>
/// Initializes a new instance of the Bar class.
/// </summary>

->

/// <summary>
/// Initializes a new instance of the <see cref="Foo"/> class.
/// </summary>

[sharwell]

💡 Don't assume the result of as is non-null. Either use a normal cast instead, or include a null check.

[sharwell]

We've had some problems in the past where failing to specify getInnermostNodeForTie: true resulted in exceptions when running the code fix on documents with syntax errors. In addition, the as XmlNodeSyntax appears to be unnecessary. I recommend the following:

SyntaxNode node = root.FindNode(diagnostic.Location.SourceSpan, findInsideTrivia: true, getInnermostNodeForTie: true);

[pdelvo]

@sharwell i implemented your feedback