Fix dlang/dmd!21247 - DMD hangs in CTFE (dlang/dmd!21248)

Fixes dlang/dmd#21247

An infinite loop was introduced by a previous refactoring.
Ensure the loop properly terminates and add tests cases.

Author: drpriver

dmd/expressionsem.d

@@ -6978,10 +6978,10 @@ private extern (C++) final class ExpressionSemanticVisitor : Visitor
         while (1)
         {
             AttribDeclaration ad = s.isAttribDeclaration();
-            if (!ad)
-                break;
-            if (ad.decl && ad.decl.length == 1)
+            if (ad && ad.decl && ad.decl.length == 1)
                 s = (*ad.decl)[0];
+            else
+                break;
         }
 
         //printf("inserting '%s' %p into sc = %p\n", s.toChars(), s, sc);

tests/dmd/fail_compilation/test21247.d

@@ -0,0 +1,20 @@
+/*
+TEST_OUTPUT:
+---
+fail_compilation/test21247.d(13): Error: anonymous union can only be a part of an aggregate, not function `hang_dmd`
+fail_compilation/test21247.d(17): Error: undefined identifier `u`
+fail_compilation/test21247.d(18): Error: undefined identifier `b`
+fail_compilation/test21247.d(20):        called from here: `hang_dmd(0u)`
+---
+ */
+// https://github.com/dlang/dmd/issues/21247
+ubyte[4] hang_dmd(uint a)
+{
+    union {
+        uint u = void;
+        ubyte[4] b;
+    }
+    u = a;
+    return b;
+}
+enum T = hang_dmd(0);

tests/dmd/fail_compilation/test21247b.d

@@ -0,0 +1,14 @@
+/*
+TEST_OUTPUT:
+---
+fail_compilation/test21247b.d(10): Error: anonymous union can only be a part of an aggregate, not function `test21247`
+---
+ */
+// https://github.com/dlang/dmd/issues/21247
+void test21247()
+{
+    union {
+        uint u = void;
+        ubyte[4] b;
+    }
+}