proc_macro: don't panic parsing ..= (fix #47950)

src/libproc_macro/lib.rs

@@ -685,7 +685,7 @@ fn joint(first: char, rest: Token, is_joint: bool, span: &mut syntax_pos::Span,
                 })
             }
 
-            DotEq => unreachable!(),
+            DotEq => joint!('.', Eq),
             OpenDelim(..) | CloseDelim(..) => unreachable!(),
             Whitespace | Comment | Shebang(..) | Eof => unreachable!(),
         };