[swift-version] Allow swift-version 4 and tests

The recent @escaping on variadic argument closures back-compat fix is
the first Swift 3.0 compatibility behavior that we don't want to carry
forwards indefinitely into the future. To address this, we
version-gate the diagnostic suppression.

Makes it an official compatibility check. Creates new test directory
for compatibility testing. Allow -swift-version 4 so that we can test
it both ways.

include/swift/Basic/Version.h

@@ -95,6 +95,9 @@ public:
   /// are attempting to maintain backward-compatibility support for.
   bool isValidEffectiveLanguageVersion() const;
 
+  /// Whether this version is in the Swift 3 family
+  bool isVersion3() const { return !empty() && Components[0] == 3; }
+
   /// Parse a version in the form used by the _compiler_version \#if condition.
   static Version parseCompilerVersionString(StringRef VersionString,
                                             SourceLoc Loc,