Remove LitKind::synthesize_token_lit.

It has a single call site in the HIR pretty printer, where the resulting
token lit is immediately converted to a string.

This commit replaces `LitKind::synthesize_token_lit` with a `Display`
impl for `LitKind`, which can be used by the HIR pretty printer.

compiler/rustc_hir_pretty/src/lib.rs

@@ -1256,7 +1256,7 @@ impl<'a> State<'a> {
 
     fn print_literal(&mut self, lit: &hir::Lit) {
         self.maybe_print_comment(lit.span.lo());
-        self.word(lit.node.synthesize_token_lit().to_string())
+        self.word(lit.node.to_string())
     }
 
     fn print_inline_asm(&mut self, asm: &hir::InlineAsm<'_>) {