Fix VB member body span used to determine whether speculative binding should be used

To determine whether or not speculative binding should be used, GetSemanticModelForSpanAsync calls into ISyntaxFactsService.GetMemberBodySpanForSpeculativeBinding(). In VB, the span returned by this method started from the end of the block statement to the start of the end block statement. However, for VB, the span should start after the block statement's terminator. Otherwise, a member body speculative semantic model will likey be returned when the requested span is within the trivia after the block statement but before the statement terminator. E.g.

Sub M() |

End Sub

src/EditorFeatures/VisualBasicTest/Completion/CompletionProviders/KeywordCompletionProviderTests.vb

@@ -173,5 +173,23 @@ End Class
 
             VerifyNoItemsExist(code)
         End Sub
+
+        <WorkItem(4167, "https://github.com/dotnet/roslyn/issues/4167")>
+        <Fact(), Trait(Traits.Feature, Traits.Features.KeywordRecommending)>
+        Public Sub ImplementsAfterSub()
+            Dim code = "
+Interface I
+End Interface
+
+Class C
+    Implements I
+
+    Sub M() $$
+    End Sub
+End Class
+"
+
+            VerifyItemExists(code, "Implements")
+        End Sub
     End Class
 End Namespace

src/Workspaces/VisualBasic/Portable/LanguageServices/VisualBasicSyntaxFactsService.vb

@@ -662,7 +662,17 @@ Namespace Microsoft.CodeAnalysis.VisualBasic
                     Return Nothing
                 End If
 
-                Return TextSpan.FromBounds(method.BlockStatement.Span.End, method.EndBlockStatement.SpanStart)
+                ' We don't want to include the BlockStatement or any trailing trivia up to and including its statement
+                ' terminator in the span. Instead, we use the start of the first statement's leading trivia (if any) up
+                ' to the start of the EndBlockStatement. If there aren't any statements in the block, we use the start
+                ' of the EndBlockStatements leading trivia.
+
+                Dim firstStatement = method.Statements.FirstOrDefault()
+                Dim spanStart = If(firstStatement IsNot Nothing,
+                                   firstStatement.FullSpan.Start,
+                                   method.EndBlockStatement.FullSpan.Start)
+
+                Return TextSpan.FromBounds(spanStart, method.EndBlockStatement.SpanStart)
             End If
 
             Return Nothing