Rename kw::Invalid -> kw::Empty

See https://rust-lang.zulipchat.com/#narrow/stream/182449-t-compiler.2Fhelp/topic/Is.20there.20a.20symbol.20for.20the.20empty.20string.3F/near/220054471
for context.

Author: jyn514

clippy_lints/src/lifetimes.rs

@@ -501,7 +501,7 @@ impl<'tcx> Visitor<'tcx> for BodyLifetimeChecker {
 
     // for lifetimes as parameters of generics
     fn visit_lifetime(&mut self, lifetime: &'tcx Lifetime) {
-        if lifetime.name.ident().name != kw::Invalid && lifetime.name.ident().name != kw::StaticLifetime {
+        if lifetime.name.ident().name != kw::Empty && lifetime.name.ident().name != kw::StaticLifetime {
             self.lifetimes_used_in_body = true;
         }
     }