[CodeGenCXX] Fix ItaniumCXXABI::getAlignmentOfExnObject to return 8-byte

alignment on Darwin.

Itanium C++ ABI specifies that _Unwind_Exception should be double-word
aligned (16B). To conform to the ABI, libraries implementing exception
handling declare the struct with __attribute__((aligned)), which aligns
the unwindHeader field (and the end of __cxa_exception) to the default
target alignment (which is typically 16-bytes).

struct __cxa_exception {
  ...
  // struct is declared with __attribute__((aligned)).
  _Unwind_Exception unwindHeader;
};

Based on the assumption that _Unwind_Exception is declared with
__attribute__((aligned)), ItaniumCXXABI::getAlignmentOfExnObject returns
the target default alignment for __attribute__((aligned)). It turns out
that libc++abi, which is used on Darwin, doesn't declare the struct with
the attribute and therefore doesn't guarantee that unwindHeader is
aligned to the alignment specified by the ABI, which in some cases
causes the program to crash because of unaligned memory accesses.

This commit avoids crashes due to unaligned memory accesses by having
getAlignmentOfExnObject return an 8-byte alignment on Darwin. I've only
fixed the problem for Darwin, but we should also figure out whether other
platforms using libc++abi need similar fixes.

rdar://problem/25314277

Differential revision: http://reviews.llvm.org/D18479

llvm-svn: 264998