[clang][CodeGen] Check initializer of zero-size fields for nullptr (#109271)

In #96422 we started treating
empty records as zero-sized for the purpose of layout. In `C`, empty
fields were never considered `isZeroSize`, so we would never have tried
to call `Init->hasSideEffects` on them. But since
#96422 we can get here when
compiling `C`, but `Init` need not exist. This patch adds a null-check
to account for this situtation.

Author: Michael137

clang/lib/CodeGen/CGExprConstant.cpp

@@ -738,7 +738,7 @@ bool ConstStructBuilder::Build(const InitListExpr *ILE, bool AllowOverwrite) {
     // Zero-sized fields are not emitted, but their initializers may still
     // prevent emission of this struct as a constant.
     if (isEmptyFieldForLayout(CGM.getContext(), Field)) {
-      if (Init->HasSideEffects(CGM.getContext()))
+      if (Init && Init->HasSideEffects(CGM.getContext()))
         return false;
       continue;
     }

clang/test/CodeGen/union-init2.c

@@ -1,4 +1,5 @@
 // RUN: %clang_cc1 -emit-llvm %s -o - -triple i686-pc-linux-gnu | FileCheck %s
+// RUN: %clang_cc1 -x c++ %s -emit-llvm -triple x86_64-linux-gnu -o - | FileCheck %s --check-prefixes=CHECK-CXX
 
 // Make sure we generate something sane instead of a ptrtoint
 // CHECK: @r, [4 x i8] undef
@@ -11,3 +12,10 @@ union z {
   long long b;
 };
 union z y = {};
+
+// CHECK: @foo = {{.*}}global %union.Foo undef, align 1
+// CHECK-CXX: @foo = {{.*}}global %union.Foo undef, align 1
+union Foo {
+  struct Empty {} val;
+};
+union Foo foo = {};