Improve hasAnnotation check for missing annotation classes

getSymbolForName returns null if the class doesn't exist,
Symtab.enterClass speculatively enters it in the symbol table even if it
isn't on the classpath.

MOE_MIGRATED_REVID=125219502

core/src/main/java/com/google/errorprone/util/ASTHelpers.java

@@ -46,6 +46,7 @@
 import com.sun.tools.javac.code.Scope;
 import com.sun.tools.javac.code.Symbol;
 import com.sun.tools.javac.code.Symbol.ClassSymbol;
+import com.sun.tools.javac.code.Symbol.CompletionFailure;
 import com.sun.tools.javac.code.Symbol.MethodSymbol;
 import com.sun.tools.javac.code.Symbol.PackageSymbol;
 import com.sun.tools.javac.code.Symbol.TypeSymbol;
@@ -485,7 +486,13 @@ public static MethodSymbol findSuperMethod(MethodSymbol method, Types types) {
    * @param annotationType The type of the annotation to look for (e.g, "javax.annotation.Nullable")
    */
   public static boolean hasAnnotation(Symbol sym, String annotationType, VisitorState state) {
-    Symbol annotationSym = state.getSymbolFromString(annotationType);
+    Symbol annotationSym = state.getSymtab().enterClass(state.getName(annotationType));
+    try {
+      annotationSym.complete();
+    } catch (CompletionFailure e) {
+      // @Inherited won't work if the annotation isn't on the classpath, but we can still check
+      // if it's present directly
+    }
     Symbol inheritedSym = state.getSymtab().inheritedType.tsym;
 
     if ((sym == null) || (annotationSym == null)) {