Check attributes on struct expression fields.

Attributes on struct expression fields were not being checked for
validity. This adds the fields as HIR nodes so that `CheckAttrVisitor`
can visit those nodes to check their attributes.

compiler/rustc_hir/src/target.rs

@@ -57,6 +57,7 @@ pub enum Target {
     MacroDef,
     Param,
     PatField,
+    ExprField,
 }
 
 impl Display for Target {
@@ -185,6 +186,7 @@ impl Target {
             Target::MacroDef => "macro def",
             Target::Param => "function param",
             Target::PatField => "pattern field",
+            Target::ExprField => "struct field",
         }
     }
 }