Improve error message for ExposedPrivateType

The diagnostic message now includes the name of the non-private member that is exposing a private type

PiperOrigin-RevId: 899711391

Author: cushon

core/src/main/java/com/google/errorprone/bugpatterns/ExposedPrivateType.java

@@ -176,7 +176,8 @@ private Description report(Tree tree, VisitorState state) {
       description.addFix(increaseVisibility.build());
       description.setMessage(
           String.format(
-              "Signatures of non-private members should not reference private classes: %s",
+              "Signature of non-private member '%s' should not reference private classes: %s",
+              getSymbol(tree).getSimpleName(),
               found.stream().map(s -> s.getSimpleName()).collect(joining(", "))));
       return description.build();
     }

core/src/test/java/com/google/errorprone/bugpatterns/ExposedPrivateTypeTest.java

@@ -40,7 +40,7 @@ public void publicFieldExposingPrivateType_fails() {
             class Outer {
               private static class PrivateInner {}
 
-              // BUG: Diagnostic contains: PrivateInner
+              // BUG: Diagnostic contains: non-private member 'field' should not reference private classes: PrivateInner
               public PrivateInner field;
             }
             """)