Update XML output for extensions

[BillWagner]

This PR updates the sample code and text output to match the current compiler output.

[BillWagner]

@jcouv This changes matches what I'm seeing in the latest compiler. Can you verify?

[BillWagner]

/cc @huangmin-ms

[jcouv]

This is the wrong expectation. The correct output is <member name="M:ClassLibrary6.E.&lt;&gt;E__0.AddValue(System.Int32)">.

The same comment applies elsewhere: the extension block should appear with its synthesized name (<>E__ followed by an index) and the synthesized name should be escaped (fix pending).

[jcouv]

The metadata design for extensions was updated to use content-based grouping and marker type names. Here's an updated example showing the impact on xml docs:

    [Fact]
    public void XmlDoc_01()
    {
        // Instance members
        var src = """
/// <summary>Summary for E</summary>
static class E
{
    /// <summary>Summary for extension block</summary>
    /// <typeparam name="T">Description for T</typeparam>
    /// <param name="t">Description for t</param>
    extension<T>(T t)
    {
        /// <summary>Summary for M</summary>
        /// <typeparam name="U">Description for U</typeparam>
        /// <param name="u">Description for u</param>
        public void M<U>(U u) => throw null!;

        /// <summary>Summary for P</summary>
        public int P => 0;
    }
}
""";
        var comp = CreateCompilation(src, parseOptions: TestOptions.RegularPreviewWithDocumentationComments, assemblyName: "Test");
        comp.VerifyEmitDiagnostics();

        var e = comp.GetMember<NamedTypeSymbol>("E");
        AssertEx.Equal("""
<member name="T:E">
    <summary>Summary for E</summary>
</member>

""", e.GetDocumentationCommentXml());

        var extension = e.GetTypeMembers().Single();
        AssertEx.Equal("T:E.<G>$8048A6C8BE30A622530249B904B537EB.<M>$D1693D81A12E8DED4ED68FE22D9E856F", extension.GetDocumentationCommentId());
        AssertEx.Equal("""
<member name="T:E.&lt;G&gt;$8048A6C8BE30A622530249B904B537EB.&lt;M&gt;$D1693D81A12E8DED4ED68FE22D9E856F">
    <summary>Summary for extension block</summary>
    <typeparam name="T">Description for T</typeparam>
    <param name="t">Description for t</param>
</member>

""", extension.GetDocumentationCommentXml());

        var mSkeleton = extension.GetMember<MethodSymbol>("M");
        AssertEx.Equal("""
<member name="M:E.&lt;G&gt;$8048A6C8BE30A622530249B904B537EB.M``1(``0)">
    <summary>Summary for M</summary>
    <typeparam name="U">Description for U</typeparam>
    <param name="u">Description for u</param>
</member>

""", mSkeleton.GetDocumentationCommentXml());

        var mImplementation = e.GetMember<MethodSymbol>("M");
        AssertEx.Equal("""
<member name="M:E.M``2(``0,``1)">
    <inheritdoc cref="M:E.&lt;G&gt;$8048A6C8BE30A622530249B904B537EB.M``1(``0)"/>
</member>

""", mImplementation.GetDocumentationCommentXml());

        var p = extension.GetMember<PropertySymbol>("P");
        AssertEx.Equal("""
<member name="P:E.&lt;G&gt;$8048A6C8BE30A622530249B904B537EB.P">
    <summary>Summary for P</summary>
</member>

""", p.GetDocumentationCommentXml());

        var pGetImplementation = e.GetMember<MethodSymbol>("get_P");
        AssertEx.Equal("""
<member name="M:E.get_P``1(``0)">
    <inheritdoc cref="P:E.&lt;G&gt;$8048A6C8BE30A622530249B904B537EB.P"/>
</member>

""", pGetImplementation.GetDocumentationCommentXml());

        var expected = """
<?xml version="1.0"?>
<doc>
    <assembly>
        <name>Test</name>
    </assembly>
    <members>
        <member name="T:E">
            <summary>Summary for E</summary>
        </member>
        <member name="M:E.M``2(``0,``1)">
            <inheritdoc cref="M:E.&lt;G&gt;$8048A6C8BE30A622530249B904B537EB.M``1(``0)"/>
        </member>
        <member name="M:E.get_P``1(``0)">
            <inheritdoc cref="P:E.&lt;G&gt;$8048A6C8BE30A622530249B904B537EB.P"/>
        </member>
        <member name="T:E.&lt;G&gt;$8048A6C8BE30A622530249B904B537EB.&lt;M&gt;$D1693D81A12E8DED4ED68FE22D9E856F">
            <summary>Summary for extension block</summary>
            <typeparam name="T">Description for T</typeparam>
            <param name="t">Description for t</param>
        </member>
        <member name="M:E.&lt;G&gt;$8048A6C8BE30A622530249B904B537EB.M``1(``0)">
            <summary>Summary for M</summary>
            <typeparam name="U">Description for U</typeparam>
            <param name="u">Description for u</param>
        </member>
        <member name="P:E.&lt;G&gt;$8048A6C8BE30A622530249B904B537EB.P">
            <summary>Summary for P</summary>
        </member>
    </members>
</doc>
""";
        AssertEx.Equal(expected, GetDocumentationCommentText(comp));

        var tree = comp.SyntaxTrees.Single();
        var model = comp.GetSemanticModel(tree);
        AssertEx.SequenceEqual(["(T, T)", "(t, T t)", "(U, U)", "(u, U u)"], PrintXmlNameSymbols(tree, model));
    }

[BillWagner]

Converting to draft. I'll update after dotnet/roslyn#78735 gets merged.