[PATCH] D12821: Allow for C's "writing off the end" idiom in __builtin_object_size

[George Burgess IV via cfe-commits]

george.burgess.iv created this revision.
george.burgess.iv added a reviewer: rsmith.
george.burgess.iv added subscribers: cfe-commits, mzolotukhin.

In C, a common idiom is:

```
struct Foo { int a; char cs[1] };
struct Foo *F = (struct Foo *)malloc(sizeof(Foo) + strlen(SomeString));
strcpy(F->cs, SomeString);
```

Currently, __builtin_object_size does not allow for this, which breaks some existing code. This patch makes us answer conservatively in Clang if the following conditions are met:

- Type is 1 or 3
- The Base is invalid/can't be determined
- The subobject we're referencing is the last subobject in the struct
- The subobject we're referencing is an array with 0 or 1 elements (for 0 elements, both `char foo[]` and `char foo[0]` syntaxes are supported)

http://reviews.llvm.org/D12821

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

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