Why is there a generic folder icon for Library in the "Go" menu?
Unfortunately, there is no Sidebar icon for the Library folder in Yosemite and El Capitan. The sidebar icons are used for the Go menu items as well.

The icon that was used in the past in the  (Mavericks and before), was (to my knowledge) ToolbarLibraryFolderIcon.icns, which does not exist anymore.

You can view the Sidebar icons that do exist in /System/Library/CoreServices/CoreTypes.bundle/Contents/Resources. They are named in the format Sidebar&lt;FOLDERNAME&gt;Folder.icns.

Interestingly, the Library folder icon (for the folder itself in Finder) does still exist, as LibraryFolderIcon.icns