Add -fobjc-tight-layout.

 - This implements gcc style Objective-C interface layout (I
   think). Currently it is always off, there is no functionality
   change unless this is passed.
   
   For the curious, the deal is that gcc lays out the fields of a
   subclass as if they were part of the superclass. That is, the
   subclass fields immediately follow the super class fields instead
   of being padded to the alignment of the superclass structure.

 - Currently gcc uses the tight layout in 32-bit and 64-bit modes, and
   llvm-gcc uses it in 32-bit only, for reasons which aren't clear
   yet. We probably want to switch to matching gcc, once this makes it
   through testing... my hope is that we can also fix llvm-gcc in
   order to maintain compatibility between the compilers.

llvm-svn: 70827