Fix SourcepathIgnoringInvocationHandler on Java 9

Use the method provided by the proxy API instead of
trying to look up the same method on the delegate's
class. It is unclear why this was done in the first place.
Trying to access the method from the delegate lead to
illegal access exceptions on certain JDKs.

subprojects/language-java/src/main/java/org/gradle/api/internal/tasks/compile/reflect/SourcepathIgnoringInvocationHandler.java

@@ -37,7 +37,6 @@ public SourcepathIgnoringInvocationHandler(Object proxied) {
     @Override
     @SuppressWarnings("unchecked")
     public Object invoke(Object proxy, Method method, Object[] args) throws Throwable {
-        Method m = proxied.getClass().getMethod(method.getName(), method.getParameterTypes());
         if (method.getName().equals(HAS_LOCATION_METHOD)) {
             // There is currently a requirement in the JDK9 javac implementation
             // that when javac is invoked with an explicitly empty sourcepath
@@ -62,6 +61,6 @@ public Object invoke(Object proxy, Method method, Object[] args) throws Throwabl
                 ((Set<JavaFileObject.Kind>) args[2]).remove(JavaFileObject.Kind.SOURCE);
             }
         }
-        return m.invoke(proxied, args);
+        return method.invoke(proxied, args);
     }
 }