Fix an oversight in GetXcodeContentsDirectory()

[aprantl]

Summary

Since FindXcodeContentsDirectoryInPath expects the *.app/Contents and DEVELOPER_DIR is supposed to point to Xcode.app, we need to append the Contents path first.

lldb/source/Host/macosx/objcxx/HostInfoMacOSX.mm

if (FileSpec fspec = HostInfo::GetShlibDir()) {
             XcodeSDK::FindXcodeContentsDirectoryInPath(fspec.GetPath());
         if (!xcode_contents_dir.empty()) {
           g_xcode_contents_path = FileSpec(xcode_contents_dir);
           return;
         }
       }
     }
 
-    std::string env_developer_dir = GetEnvDeveloperDir();
+    llvm::SmallString<128> env_developer_dir(GetEnvDeveloperDir());
     if (!env_developer_dir.empty()) {
-      // FIXME: This looks like it couldn't possibly work!
+      llvm::sys::path::append(env_developer_dir, "Contents");
       std::string xcode_contents_dir =
           XcodeSDK::FindXcodeContentsDirectoryInPath(env_developer_dir);
       if (!xcode_contents_dir.empty()) {
         g_xcode_contents_path = FileSpec(xcode_contents_dir);
         return;
       }
     }