Edit & Continue support for field access in auto-properties (#78289)

Fixes #77951

Field access in auto-properties mostly already worked in EnC.

- Update the EnC unit testing infrastructure to always run C# tests with
the "preview" language version and VB tests with the "latest" language
version.
- Add unit test coverage
- Fix small issue in the `AbstractEditAndContinueAnalyzer` to allow
auto-properties to be detected from a set accessor.

cc @tmat

Author: DustinCampbell

src/Compilers/CSharp/Test/Emit2/Emit/EditAndContinue/EditAndContinueTests.cs

@@ -2199,6 +2199,52 @@ class C
             capabilities: EditAndContinueCapabilities.AddMethodToExistingType);
     }
 
+    [Fact]
+    public void Property_Update_ExpressionBodyToAutoProp_FieldAccess()
+    {
+        var src1 = @"
+class C
+{
+    public int P => <AS:0>1</AS:0>;
+}
+";
+        var src2 = @"
+class C
+{
+    public int P => <AS:0>field</AS:0>;
+}
+";
+        var edits = GetTopEdits(src1, src2);
+        var active = GetActiveStatements(src1, src2);
+
+        edits.VerifySemanticDiagnostics(
+            active,
+            capabilities: EditAndContinueCapabilities.AddInstanceFieldToExistingType);
+    }
+
+    [Fact]
+    public void Property_Update_Accessor_ExpressionBodyToAutoProp_FieldAccess()
+    {
+        var src1 = @"
+class C
+{
+    public int P { get => <AS:0>1</AS:0>; }
+}
+";
+        var src2 = @"
+class C
+{
+    public int P { get => <AS:0>field</AS:0>; }
+}
+";
+        var edits = GetTopEdits(src1, src2);
+        var active = GetActiveStatements(src1, src2);
+
+        edits.VerifySemanticDiagnostics(
+            active,
+            capabilities: EditAndContinueCapabilities.AddInstanceFieldToExistingType);
+    }
+
     [Fact]
     public void Property_Auto_Record_ReplacingNonPrimaryWithPrimary_Getter()
     {

src/Features/CSharpTest/EditAndContinue/ActiveStatementTests.cs

@@ -17,6 +17,7 @@ internal sealed class CSharpEditAndContinueTestVerifier(Action<SyntaxNode>? faul
     public override string ProjectFileExtension => ".csproj";
     public override TreeComparer<SyntaxNode> TopSyntaxComparer => SyntaxComparer.TopLevel;
     public override string? TryGetResource(string keyword) => EditingTestBase.TryGetResource(keyword);
+    public override ParseOptions ParseOptions => CSharpParseOptions.Default.WithLanguageVersion(LanguageVersion.Preview);
 
     public override ImmutableArray<SyntaxNode> GetDeclarators(ISymbol method)
     {

src/Features/CSharpTest/EditAndContinue/Helpers/CSharpEditAndContinueTestVerifier.cs

@@ -134,7 +134,7 @@ internal static string GetDocumentFilePath(int documentIndex)
     private static SyntaxTree ParseSource(string markedSource, int documentIndex = 0)
         => SyntaxFactory.ParseSyntaxTree(
             SourceMarkers.Clear(markedSource),
-            CSharpParseOptions.Default.WithLanguageVersion(LanguageVersion.CSharp12),
+            CSharpParseOptions.Default.WithLanguageVersion(LanguageVersion.Preview),
             path: GetDocumentFilePath(documentIndex));
 
     internal static EditScriptDescription GetTopEdits(string methodBody1, string methodBody2, MethodKind kind)