Map TextInput textContentType strings to Objective-C constants (#22611)

Summary:
This is an updated version of #22579 which uses compile conditionals to prevent `use of undeclared identifier` errors when compiling on older versions of Xcode.

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Currently the only `textContentType` values that work are: `username`, `password`, `location`, `name` and `nickname`. This is due to the strings provided by React Native not matching up with the underlying string constants used in iOS (with the exception of the aforementioned types). Issue #22578 has more detail examples/explanation.
Pull Request resolved: https://github.com/facebook/react-native/pull/22611

Differential Revision: D13460949

Pulled By: cpojer

fbshipit-source-id: e6d1108422b850ebc3aea05693ed05118b77b5de

RNTester/js/TextInputExample.ios.js

@@ -1085,4 +1085,19 @@ exports.examples = [
       );
     },
   },
+  {
+    title: 'Text Content Type',
+    render: function() {
+      return (
+        <View>
+          <WithLabel label="emailAddress">
+            <TextInput textContentType="emailAddress" style={styles.default} />
+          </WithLabel>
+          <WithLabel label="name">
+            <TextInput textContentType="name" style={styles.default} />
+          </WithLabel>
+        </View>
+      );
+    },
+  },
 ];