Convert some functions to return Cow<'static,str> instead of String to reduce potential reallocations

compiler/rustc_parse/src/parser/expr.rs

@@ -1528,7 +1528,7 @@ impl<'a> Parser<'a> {
             .span_suggestion(
                 token.span,
                 "must have an integer part",
-                pprust::token_to_string(token),
+                pprust::token_to_string(token).into(),
                 Applicability::MachineApplicable,
             )
             .emit();