[clang] Add zero-initialization for fixed point types (#80781)

Author: PiJoules

clang/lib/AST/ExprConstant.cpp

@@ -11427,6 +11427,10 @@ class FixedPointExprEvaluator
     return true;
   }
 
+  bool ZeroInitialization(const Expr *E) {
+    return Success(0, E);
+  }
+
   //===--------------------------------------------------------------------===//
   //                            Visitor Methods
   //===--------------------------------------------------------------------===//

clang/test/AST/fixed-point-zero-init.cpp

@@ -0,0 +1,5 @@
+// RUN: %clang_cc1 -ffixed-point %s -verify
+// expected-no-diagnostics
+
+constexpr _Accum a[2] = {};
+static_assert(a[0] == 0 && a[0] != 1);