Fix DecompilerSettings.PatternMatching should only be active in case >= CS70.

ICSharpCode.Decompiler.Tests/PrettyTestRunner.cs

@@ -486,7 +486,7 @@ namespace ICSharpCode.Decompiler.Tests
 		[Test]
 		public void Issue1080([ValueSource(nameof(roslynOnlyOptions))] CompilerOptions cscOptions)
 		{
-			RunForLibrary(cscOptions: cscOptions);
+			RunForLibrary(cscOptions: cscOptions, decompilerSettings: new DecompilerSettings(CSharp.LanguageVersion.CSharp6));
 		}
 
 		[Test]

ICSharpCode.Decompiler.Tests/TestCases/Pretty/ShortCircuit.cs

@@ -320,15 +320,9 @@ namespace ICSharpCode.Decompiler.Tests.TestCases.Pretty
 			if (!F(1))
 			{
 			}
-			if (F(2) && F(3))
-			{
-			}
 			if (F(4) || F(5))
 			{
 			}
-			if (F(0) && F(1) && !F(2) && (F(3) || F(4)))
-			{
-			}
 			E();
 		}
 #endif

ICSharpCode.Decompiler/DecompilerSettings.cs

@@ -104,6 +104,7 @@ namespace ICSharpCode.Decompiler
 				discards = false;
 				localFunctions = false;
 				deconstruction = false;
+				patternMatching = false;
 			}
 			if (languageVersion < CSharp.LanguageVersion.CSharp7_2)
 			{
@@ -157,7 +158,7 @@ namespace ICSharpCode.Decompiler
 				return CSharp.LanguageVersion.CSharp7_2;
 			// C# 7.1 missing
 			if (outVariables || throwExpressions || tupleTypes || tupleConversions
-				|| discards || localFunctions)
+				|| discards || localFunctions || deconstruction || patternMatching)
 				return CSharp.LanguageVersion.CSharp7;
 			if (awaitInCatchFinally || useExpressionBodyForCalculatedGetterOnlyProperties || nullPropagation
 				|| stringInterpolation || dictionaryInitializers || extensionMethodsInCollectionInitializers