Match against attribute name when validating attributes

Extract attribute name once and match it against symbols that are being
validated, instead of using `Session::check_name` for each symbol
individually.

Assume that all validated attributes are used, instead of marking them
as such, since the attribute check should be exhaustive.

compiler/rustc_ast/src/ast.rs

@@ -100,6 +100,7 @@ pub struct Path {
 }
 
 impl PartialEq<Symbol> for Path {
+    #[inline]
     fn eq(&self, symbol: &Symbol) -> bool {
         self.segments.len() == 1 && { self.segments[0].ident.name == *symbol }
     }