Support @CompileTimeConstant for enum fields.

The support for *class* fields was introduced in unknown commit. This CL merely extends it to enum fields, considering the similarity between them. There is also a feature request b/149290598 filed a few years back.

Specifically, this CL aims to support the following use case (taken from b/149290598):
```
public class Example {

  public enum ExampleEnum {
    A("a"), B("b");

    @CompileTimeConstant final String s;

    ExampleEnum(@CompileTimeConstant final String s) {
      this.s = s;
    }

    void invokeCtcMethod() {
      ctcMethod(s);
    }

    private void ctcMethod(@CompileTimeConstant String string) {}
  }
}
```

PiperOrigin-RevId: 529230341

check_api/src/main/java/com/google/errorprone/matchers/CompileTimeConstantExpressionMatcher.java

@@ -131,11 +131,12 @@ protected Boolean defaultAction(Tree node, Void unused) {
               Symbol.VarSymbol varSymbol = (Symbol.VarSymbol) getSymbol(node);
               Symbol owner = varSymbol.owner;
               ElementKind ownerKind = owner.getKind();
-              // Check that the identifier is a formal method/constructor parameter or a class
+              // Check that the identifier is a formal method/constructor parameter, or a class/enum
               // field.
               if (ownerKind != ElementKind.METHOD
                   && ownerKind != ElementKind.CONSTRUCTOR
-                  && ownerKind != ElementKind.CLASS) {
+                  && ownerKind != ElementKind.CLASS
+                  && ownerKind != ElementKind.ENUM) {
                 return false;
               }
               // Check that the symbol is final

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

@@ -238,7 +238,8 @@ public Description matchAssignment(AssignmentTree node, VisitorState state) {
     if (assignedSymbol == null || assignedSymbol.owner == null) {
       return Description.NO_MATCH;
     }
-    if (assignedSymbol.owner.getKind() != ElementKind.CLASS) {
+    if (assignedSymbol.owner.getKind() != ElementKind.CLASS
+        && assignedSymbol.owner.getKind() != ElementKind.ENUM) {
       return Description.NO_MATCH;
     }
     if (!hasCompileTimeConstantAnnotation(state, assignedSymbol)) {

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

@@ -570,6 +570,76 @@ public void noDiagnostic_whenInvokingMethodWithFinalField() {
         .doTest();
   }
 
+  @Test
+  public void reportsDiagnostic_whenConstantEnumFieldDeclaredWithoutFinal() {
+    compilationHelper
+        .addSourceLines(
+            "Test.java",
+            "import com.google.errorprone.annotations.CompileTimeConstant;",
+            "public enum Test {",
+            "  A(\"A\");",
+            "  // BUG: Diagnostic contains: . Did you mean to make 's' final?",
+            "  @CompileTimeConstant String s;",
+            "  Test(@CompileTimeConstant String s) {",
+            "    this.s = s;",
+            "  }",
+            "}")
+        .doTest();
+  }
+
+  @Test
+  public void noDiagnostic_whenConstantEnumFieldDeclaredFinal() {
+    compilationHelper
+        .addSourceLines(
+            "Test.java",
+            "import com.google.errorprone.annotations.CompileTimeConstant;",
+            "public enum Test {",
+            "  A(\"A\");",
+            "  @CompileTimeConstant final String s;",
+            "  Test(@CompileTimeConstant String s) {",
+            "    this.s = s;",
+            "  }",
+            "}")
+        .doTest();
+  }
+
+  @Test
+  public void reportsDiagnostic_whenInitialisingFinalEnumFieldWithNonConstant() {
+    compilationHelper
+        .addSourceLines(
+            "Test.java",
+            "import com.google.errorprone.annotations.CompileTimeConstant;",
+            "public enum Test {",
+            "  A(\"A\");",
+            "  @CompileTimeConstant final String s;",
+            "  Test(String s) {",
+            "    // BUG: Diagnostic contains: Non-compile-time constant expression",
+            "    this.s = s;",
+            "  }",
+            "}")
+        .doTest();
+  }
+
+  @Test
+  public void noDiagnostic_whenInvokingMethodWithFinalEnumField() {
+    compilationHelper
+        .addSourceLines(
+            "Test.java",
+            "import com.google.errorprone.annotations.CompileTimeConstant;",
+            "public enum Test {",
+            "  A(\"A\");",
+            "  @CompileTimeConstant final String s;",
+            "  Test(@CompileTimeConstant String s) {",
+            "    this.s = s;",
+            "  }",
+            "  void invokeCTCMethod() {",
+            "    ctcMethod(s);",
+            "  }",
+            "  void ctcMethod(@CompileTimeConstant String s) {}",
+            "}")
+        .doTest();
+  }
+
   @Test
   public void nonConstantField_positive() {
     compilationHelper