Disable the UnusedDeclarationsAnalyzer by default

Related to issue #1058.

The analyzer reports thousands of spurious errors when running over
Roslyn.sln. In order to move forward to newer builds of the toolset
compilers and diagnostics, it needs to be disabled.

src/Diagnostics/Roslyn/Core/Maintainability/UnusedDeclarationsAnalyzer.cs

@@ -18,7 +18,7 @@ internal abstract partial class UnusedDeclarationsAnalyzer<TLanguageKindEnum> :
 
         private static readonly LocalizableString s_messageFormat = new LocalizableResourceString(nameof(RoslynDiagnosticsResources.UnusedDeclarationsMessage), RoslynDiagnosticsResources.ResourceManager, typeof(RoslynDiagnosticsResources));
 
-        internal static readonly DiagnosticDescriptor s_rule = new DiagnosticDescriptor(RoslynDiagnosticIds.DeadCodeRuleId, s_title, s_messageFormat, Category, DiagnosticSeverity.Warning, isEnabledByDefault: true);
+        internal static readonly DiagnosticDescriptor s_rule = new DiagnosticDescriptor(RoslynDiagnosticIds.DeadCodeRuleId, s_title, s_messageFormat, Category, DiagnosticSeverity.Warning, isEnabledByDefault: false);
 
         internal static readonly DiagnosticDescriptor s_triggerRule = new TriggerDiagnosticDescriptor(RoslynDiagnosticIds.DeadCodeTriggerRuleId, WellKnownDiagnosticTags.Unnecessary, WellKnownDiagnosticTags.Telemetry);