[PATCH] D12169: Relax constexpr rules to improve __builtin_object_size's accuracy

[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.

(Hoping the formatting works as one would expect)

Motivating examples:
Pre-patch:

```
__builtin_object_size((char*)&foo, 0) != __builtin_object_size(&foo, 0) // if __builtin_object_size(&foo, 0) != -1
__builtin_object_size(&foo[1].bar[-1].baz, 1) == -1. // Always.
```

Post-patch:
Both act as one would expect. This was accomplished by making three changes:

  - Adding a flag to PointerExprEvaluator that makes it more accepting of reinterpret_casts.
  - Making array index/pointer offset less coupled in PointerExprEvaluator (we now carry around an extra Offset field that denotes how far we are away from an object boundary). 
  - Adding an OutermostMemberEvaluator that ignores `foo[1].bar[-1]` in `foo[1].bar[-1].baz`, and is more relaxed with casts/pointer arithmetic/etc. (Not 100% sold on the name)

http://reviews.llvm.org/D12169

Files:
  lib/AST/ExprConstant.cpp
  test/CXX/expr/expr.const/p2-0x.cpp
  test/CodeGen/object-size.c

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