Java2Swift: Use @_nonoverride on generated convenience initializers

Swift has inheritance of initializers, while Java does not have inheritance
of constructors. This means that Swift's attempt to treat a convenience
initializer as an override within a subclass can cause problems with
mismatched signatures. Use `@_nonoverride` to avoid this problem.

Author: DougGregor

Sources/Java2SwiftLib/JavaClassTranslator.swift

@@ -492,9 +492,10 @@ extension JavaClassTranslator {
     let throwsStr = javaConstructor.throwsCheckedException ? "throws" : ""
     let accessModifier = javaConstructor.isPublic ? "public " : ""
     let convenienceModifier = translateAsClass ? "convenience " : ""
+    let nonoverrideAttribute = translateAsClass ? "@_nonoverride " : ""
     return """
       @JavaMethod
-      \(raw: accessModifier)\(raw: convenienceModifier)init(\(raw: parametersStr))\(raw: throwsStr)
+      \(raw: nonoverrideAttribute)\(raw: accessModifier)\(raw: convenienceModifier)init(\(raw: parametersStr))\(raw: throwsStr)
       """
   }
 

Tests/Java2SwiftTests/Java2SwiftTests.swift

@@ -280,7 +280,7 @@ class Java2SwiftTests: XCTestCase {
         """,
         """
           @JavaMethod
-          public convenience init(environment: JNIEnvironment? = nil)
+          @_nonoverride public convenience init(environment: JNIEnvironment? = nil)
         """,
         """
           @JavaMethod
@@ -314,7 +314,7 @@ class Java2SwiftTests: XCTestCase {
         """,
         """
           @JavaMethod
-          public convenience init(environment: JNIEnvironment? = nil)
+          @_nonoverride public convenience init(environment: JNIEnvironment? = nil)
         """,
         """
           @JavaMethod