[clang] [clang][CGRecordLayout] Remove dependency on isZeroSize (PR #96422)

[Eli Friedman via cfe-commits]

================
@@ -1,7 +1,19 @@
-// RUN: %clang_cc1 -emit-llvm < %s | grep "zeroinitializer, i16 16877"
+// RUN: %clang_cc1 %s -emit-llvm -triple x86_64-linux-gnu -o - | FileCheck %s --check-prefixes=CHECK,EMPTY
+// RUN: %clang_cc1 %s -emit-llvm -triple x86_64-windows-msvc -o - | FileCheck %s --check-prefixes=CHECK,EMPTY-MSVC
 // PR4390
 struct sysfs_dirent {
- union { struct sysfs_elem_dir {} s_dir; };
+ union { struct sysfs_elem_dir { int x; } s_dir; };
  unsigned short s_mode;
 };
 struct sysfs_dirent sysfs_root = { {}, 16877 };
+
+// CHECK: @sysfs_root = {{.*}}global %struct.sysfs_dirent { %union.anon zeroinitializer, i16 16877 }
+
+struct Foo {
+ union { struct empty {} x; };
+ unsigned short s_mode;
+};
+struct Foo foo = { {}, 16877 };
+
+// EMPTY:      @foo = {{.*}}global %struct.Foo { i16 16877 }
+// EMPTY-MSVC: @foo = {{.*}}global %struct.Foo { [4 x i8] undef, i16 16877 }
----------------
efriedma-quic wrote:

clang doesn't currently implement the C23 rules for empty-brace initialization... but I guess if you extend those rules to empty structs/unions (which aren't valid C, but ignoring that), we arguably should zero-init?  But really, nobody should notice; this weird stuff is only realistically used for stuff like FAMs.

Testcase which MSVC accepts:

```
struct Foo {
 unsigned short s_mode;
 struct empty {int a[0];} x; 
 };
struct Foo f() { return (struct Foo){16877}; };
```

See #78034/#97121.

https://github.com/llvm/llvm-project/pull/96422