Rollup merge of #42466 - wesleywiser:fix_42407, r=arielb1

syntax_pos::Symbol should not implement Sync

Fixes #42407

src/libsyntax_pos/symbol.rs

@@ -81,6 +81,7 @@ fn decode<D: Decoder>(d: &mut D) -> Result<Ident, D::Error> {
 
 // The interner in thread-local, so `Symbol` shouldn't move between threads.
 impl !Send for Symbol { }
+impl !Sync for Symbol { }
 
 impl Symbol {
     /// Maps a string to its interned representation.