Conditionally use deprecated signature for initWithContentsOfURL

Homebrew's CI is breaking on 10.12 (Sierra), for Bazel 0.24+ due to the signature used for NSDictionary:initWithContentsOfURL introduced in the fix for bazelbuild#7371.

This signature with an error parameter is only available on 10.13+ (High Sierra). Here's Apple's documentation: https://developer.apple.com/documentation/foundation/nsdictionary/1416069-initwithcontentsofurl?language=objc

The Homebrew issue:
Homebrew/homebrew-core#38651

Closes bazelbuild#8068.

PiperOrigin-RevId: 244357959

tools/osx/xcode_locator.m

@@ -177,8 +177,16 @@ static void AddEntryToDictionary(
           url, version, expandedVersion);
 
     NSURL *versionPlistUrl = [url URLByAppendingPathComponent:@"Contents/version.plist"];
-    NSDictionary *versionPlistContents = [[NSDictionary alloc] initWithContentsOfURL:versionPlistUrl
-                                                                               error:nil];
+
+    // macOS 10.13 changed the signature of initWithContentsOfURL,
+    // and deprecated the old one.
+    NSDictionary *versionPlistContents;
+#if MAC_OS_X_VERSION_MIN_REQUIRED > MAC_OS_X_VERSION_10_12
+    versionPlistContents = [[NSDictionary alloc] initWithContentsOfURL:versionPlistUrl error:nil];
+#else
+    versionPlistContents = [[NSDictionary alloc] initWithContentsOfURL:versionPlistUrl];
+#endif
+
     NSString *productVersion = [versionPlistContents objectForKey:@"ProductBuildVersion"];
     if (productVersion) {
       expandedVersion = [expandedVersion stringByAppendingFormat:@".%@", productVersion];