[de minimis] IDE support for C# exception filters

* Add 'when' keyword recommender to the completion list
* Ensure that the completion appears inside exception filters.
* Add 'when' keyword span to keyword highlighting for try..catch statements.
* Ensure that parenthesis simplication works catch filter expressions
* Add tests for 'when' classification (it already was classified correctly) (changeset 1387214)

Src/Workspaces/CSharp/Portable/Extensions/ContextQuery/SyntaxTreeExtensions.cs

@@ -2069,7 +2069,14 @@ public static bool IsLabelContext(this SyntaxTree syntaxTree, int position, Canc
                 return true;
             }
 
-            // (SometType) |
+            // when ( |
+            if (token.CSharpKind() == SyntaxKind.OpenParenToken &&
+                token.GetPreviousToken(includeSkipped: true).CSharpKind() == SyntaxKind.WhenKeyword)
+            {
+                return true;
+            }
+
+            // (SomeType) |
             if (token.IsAfterPossibleCast())
             {
                 return true;

Src/Workspaces/CSharp/Portable/Extensions/ParenthesizedExpressionSyntaxExtensions.cs

@@ -33,6 +33,7 @@ public static bool CanRemoveParentheses(this ParenthesizedExpressionSyntax node)
             //   foreach (var y in (x)) -> foreach (var y in x)
             //   lock ((x))             -> lock (x)
             //   using ((x))            -> using (x)
+            //   catch when ((x))       -> catch when (x)
             if ((node.IsParentKind(SyntaxKind.EqualsValueClause) && ((EqualsValueClauseSyntax)node.Parent).Value == node) ||
                 (node.IsParentKind(SyntaxKind.IfStatement) && ((IfStatementSyntax)node.Parent).Condition == node) ||
                 (node.IsParentKind(SyntaxKind.ReturnStatement) && ((ReturnStatementSyntax)node.Parent).Expression == node) ||
@@ -44,7 +45,8 @@ public static bool CanRemoveParentheses(this ParenthesizedExpressionSyntax node)
                 (node.IsParentKind(SyntaxKind.ForStatement) && ((ForStatementSyntax)node.Parent).Condition == node) ||
                 (node.IsParentKind(SyntaxKind.ForEachStatement) && ((ForEachStatementSyntax)node.Parent).Expression == node) ||
                 (node.IsParentKind(SyntaxKind.LockStatement) && ((LockStatementSyntax)node.Parent).Expression == node) ||
-                (node.IsParentKind(SyntaxKind.UsingStatement) && ((UsingStatementSyntax)node.Parent).Expression == node))
+                (node.IsParentKind(SyntaxKind.UsingStatement) && ((UsingStatementSyntax)node.Parent).Expression == node) ||
+                (node.IsParentKind(SyntaxKind.CatchFilterClause) && ((CatchFilterClauseSyntax)node.Parent).FilterExpression == node))
             {
                 return true;
             }