rustc_span: Ident::invalid -> Ident::empty

The equivalent for `Symbol`s was renamed some time ago (`kw::Invalid` -> `kw::Empty`), and it makes sense to do the same thing for `Ident`s.

Author: petrochenkov

tests/ui-internal/unnecessary_symbol_str.fixed

@@ -11,6 +11,6 @@ fn main() {
     Symbol::intern("foo") == rustc_span::sym::clippy;
     Symbol::intern("foo") == rustc_span::symbol::kw::SelfLower;
     Symbol::intern("foo") != rustc_span::symbol::kw::SelfUpper;
-    Ident::invalid().name == rustc_span::sym::clippy;
-    rustc_span::sym::clippy == Ident::invalid().name;
+    Ident::empty().name == rustc_span::sym::clippy;
+    rustc_span::sym::clippy == Ident::empty().name;
 }

tests/ui-internal/unnecessary_symbol_str.rs

@@ -11,6 +11,6 @@ fn main() {
     Symbol::intern("foo").as_str() == "clippy";
     Symbol::intern("foo").to_string() == "self";
     Symbol::intern("foo").to_ident_string() != "Self";
-    &*Ident::invalid().as_str() == "clippy";
-    "clippy" == Ident::invalid().to_string();
+    &*Ident::empty().as_str() == "clippy";
+    "clippy" == Ident::empty().to_string();
 }

tests/ui-internal/unnecessary_symbol_str.stderr

@@ -26,14 +26,13 @@ LL |     Symbol::intern("foo").to_ident_string() != "Self";
 error: unnecessary `Symbol` to string conversion
   --> $DIR/unnecessary_symbol_str.rs:14:5
    |
-LL |     &*Ident::invalid().as_str() == "clippy";
-   |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ help: try: `Ident::invalid().name == rustc_span::sym::clippy`
+LL |     &*Ident::empty().as_str() == "clippy";
+   |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ help: try: `Ident::empty().name == rustc_span::sym::clippy`
 
 error: unnecessary `Symbol` to string conversion
   --> $DIR/unnecessary_symbol_str.rs:15:5
    |
-LL |     "clippy" == Ident::invalid().to_string();
-   |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ help: try: `rustc_span::sym::clippy == Ident::invalid().name`
+LL |     "clippy" == Ident::empty().to_string();
+   |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ help: try: `rustc_span::sym::clippy == Ident::empty().name`
 
 error: aborting due to 5 previous errors
-