rename ast::ViewItemExternMod to ast::ViewItemExternCrate, and clean::ExternMod to clean::ExternCrate
auto merge of #12750 : liigo/rust/De-extern-mod, r=alexcrichton

rename ast::ViewItemExternMod to ast::ViewItemExternCrate
rename rustdoc::clean::ExternMod to rustdoc::clean::ExternCrate
because `extern mod` is out.