Use DiagnosticDescriptor.GetEffectiveSeverity to check for suppressions

Closes #976

Author: sharwell

StyleCop.Analyzers/StyleCop.Analyzers/Helpers/DiagnosticOptionsHelper.cs

@@ -56,7 +56,7 @@ internal static bool IsAnalyzerSuppressed(this Compilation compilation, Diagnost
         /// <returns>True if the diagnostic is currently suppressed.</returns>
         internal static bool IsAnalyzerSuppressed(this CompilationOptions compilationOptions, DiagnosticDescriptor descriptor)
         {
-            switch (compilationOptions.SpecificDiagnosticOptions.GetValueOrDefault(descriptor.Id))
+            switch (descriptor.GetEffectiveSeverity(compilationOptions))
             {
             case ReportDiagnostic.Suppress:
                 return true;

StyleCop.Analyzers/StyleCop.Analyzers/NamingRules/SA1304NonPrivateReadonlyFieldsMustBeginWithUpperCaseLetter.cs

@@ -45,6 +45,9 @@ internal class SA1304NonPrivateReadonlyFieldsMustBeginWithUpperCaseLetter : Diag
 
         private static readonly Action<SyntaxNodeAnalysisContext> FieldDeclarationAction = HandleFieldDeclaration;
 
+        private static readonly DiagnosticDescriptor SA1307Descriptor =
+            new SA1307AccessibleFieldsMustBeginWithUpperCaseLetter().SupportedDiagnostics.Single();
+
         /// <inheritdoc/>
         public override ImmutableArray<DiagnosticDescriptor> SupportedDiagnostics { get; } =
             ImmutableArray.Create(Descriptor);
@@ -84,8 +87,7 @@ private static void HandleFieldDeclaration(SyntaxNodeAnalysisContext context)
             {
                 // SA1307 is taken precedence here. SA1307 should be reported if the field is accessible.
                 // So if SA1307 is enabled this diagnostic will only be reported for internal fields.
-                if (context.SemanticModel.Compilation.Options.SpecificDiagnosticOptions
-                    .GetValueOrDefault(SA1307AccessibleFieldsMustBeginWithUpperCaseLetter.DiagnosticId, ReportDiagnostic.Default) != ReportDiagnostic.Suppress)
+                if (!context.IsAnalyzerSuppressed(SA1307Descriptor))
                 {
                     return;
                 }