Do not add modifiers to partial methods

Partial methods cannot have explicit modifiers, they are implicitly
private.

closes #46

Author: jaredpar

src/Microsoft.DotNet.CodeFormatting.Tests/Rules/ExplicitVisibilityRuleTests.cs

@@ -360,6 +360,28 @@ private class N2 { }
 }
 
 public partial class C { }
+";
+
+            Verify(text, expected, runFormatter: false);
+        }
+
+        [Fact]
+        public void IgnorePartialMethods()
+        {
+            var text = @"
+class C
+{
+    void M1();
+    partial void M2();
+}
+";
+
+            var expected = @"
+internal class C
+{
+    private void M1();
+    partial void M2();
+}
 ";
 
             Verify(text, expected, runFormatter: false);

src/Microsoft.DotNet.CodeFormatting/Rules/ExplicitVisibilityRule.cs

@@ -68,7 +68,7 @@ public override SyntaxNode VisitConstructorDeclaration(ConstructorDeclarationSyn
 
             public override SyntaxNode VisitMethodDeclaration(MethodDeclarationSyntax node)
             {
-                if (node.ExplicitInterfaceSpecifier != null)
+                if (node.ExplicitInterfaceSpecifier != null || node.Modifiers.Any(x => x.IsKind(SyntaxKind.PartialKeyword)))
                 {
                     return node;
                 }