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.

compiler/rustc_hir/src/hir.rs

@@ -28,7 +28,7 @@ pub struct Lifetime {
     pub span: Span,
 
     /// Either "`'a`", referring to a named lifetime definition,
-    /// or "``" (i.e., `kw::Invalid`), for elision placeholders.
+    /// or "``" (i.e., `kw::Empty`), for elision placeholders.
     ///
     /// HIR lowering inserts these placeholders in type paths that
     /// refer to type definitions needing lifetime parameters,