[PATCH] D17746: Fix PR26741 -- __builtin_object_size is not consistently conservative with C++ inheritance

[George Burgess IV via cfe-commits]

george.burgess.iv created this revision.
george.burgess.iv added a reviewer: rsmith.
george.burgess.iv added a subscriber: cfe-commits.

This patch fixes PR26741, and makes us handle inheritance more sanely.

Broken code:

```
struct Foo { char a[1]; };
struct Bar : Foo {};

int break() {
  Bar *b;
  int size = __builtin_object_size(b->a, 1);
  assert(size == -1); // Fires; size is 1.
}
```

Because we're now handling inheritance, this patch has a few fun things in it (see: the new loop at ExprConstant.cpp:6489) so that we aren't overly conservative when inheritance is involved. I'm not entirely thrilled with how we determine if a base class is considered to be at the end of a derived class; better approaches are appreciated.

http://reviews.llvm.org/D17746

Files:
  lib/AST/ExprConstant.cpp
  test/CodeGen/object-size.cpp

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