Implement #pragma pack use in structure packing. The general approach

is to encode the state of the #pragma pack stack as an attribute when
the structure is declared. 

 - Extend PackedAttr to take an alignment (in bits), and reuse for
   both __attribute__((packed)) (which takes no argument, instead
   packing tightly (to "minimize the memory required") and for #pragma
   pack (which allows specification of the maximum alignment in
   bytes). __attribute__((packed)) is just encoded as Alignment=1.

   This conflates two related but different mechanisms, but it didn't
   seem worth another attribute.

 - I have attempted to follow the MSVC semantics as opposed to the gcc
   ones, since if I understand correctly #pragma pack originated with
   MSVC. The semantics are generally equivalent except when the stack
   is altered during the definition of a structure; its not clear if
   anyone does this in practice. See testcase if curious.

llvm-svn: 57623