[clang][Interp] Fix primitive lambda capture defaults

We need to use InitField here, not SetField.

Author: tbaederr

clang/lib/AST/Interp/ByteCodeExprGen.cpp

@@ -2025,7 +2025,7 @@ bool ByteCodeExprGen<Emitter>::VisitLambdaExpr(const LambdaExpr *E) {
       if (!this->visit(Init))
         return false;
 
-      if (!this->emitSetField(*T, F.Offset, E))
+      if (!this->emitInitField(*T, F.Offset, E))
         return false;
     } else {
       if (!this->emitDupPtr(E))

clang/test/AST/Interp/lambda.cpp

@@ -248,3 +248,19 @@ namespace ns2_capture_this_byval {
   constexpr auto L = S{5}.f(S{10});
   static_assert(L(S{100}) == 115, "");
 } // end test_captures_1::ns2_capture_this_byval
+
+namespace CaptureDefaults {
+  struct S {
+    int x;
+  };
+
+  constexpr auto f = [x = S{10}]() {
+      return x.x;
+  };
+  static_assert(f() == 10, "");
+
+  constexpr auto f2 = [x = 3]() {
+      return x;
+  };
+  static_assert(f2() == 3, "");
+}