[clang] [Clang] Fix 'counted_by' for nested struct pointers (PR #110497)

[Jan Hendrik Farr via cfe-commits]

Cydox wrote:

> > Yeah so the problem is if you do `__builtin_dynamic_object_size(v, 0)`
> > In that case it's a `DeclRefExpr`, a pointer, and an `LValue`.
> 
> Can you give a more complete example? I just tried the following, and I see an lvaluetorvalue cast.
> 
> ```
> int f(const void *p) { return __builtin_dynamic_object_size(p, 0); }
> ```

```C
// test2.c

#include <stdio.h>
#include <stdlib.h>
#include <unistd.h>

struct variable {
        int a;
        int b;
        int length;
        short array[] __attribute__((counted_by(length)));
};

int main(int argc, char *argv[])
{
        struct variable *v;

        v = malloc(sizeof(struct variable) + sizeof(short) * 32);
        v->length = 32;


        printf("%zu\n", __builtin_dynamic_object_size(v, 0));

        return 0;
}
```

I added this `StructBase->dump`:
```
diff --git a/clang/lib/CodeGen/CGExpr.cpp b/clang/lib/CodeGen/CGExpr.cpp
index 4aca60685f37..7a06819f1a67 100644
--- a/clang/lib/CodeGen/CGExpr.cpp
+++ b/clang/lib/CodeGen/CGExpr.cpp
@@ -1162,6 +1162,8 @@ llvm::Value *CodeGenFunction::EmitLoadOfCountedByField(
   if (!StructBase || StructBase->HasSideEffects(getContext()))
     return nullptr;
 
+  StructBase->dump();
+
   llvm::Value *Res = nullptr;
   if (StructBase->getType()->isPointerType()) {
     LValueBaseInfo BaseInfo;

```

```
$ clang test2.c
DeclRefExpr 0x34b302c8 'struct variable *' lvalue Var 0x34b2fce8 'v' 'struct variable *'

$ ./a.out
76
```

In my testing I also added a print statement that printed a dump when both `StructBase->getType()->isPointerType()` and `StructBase->isLValue()` were true and there were a bunch of hits when compiling the kernel.

https://github.com/llvm/llvm-project/pull/110497