Define portable macros for packed struct definitions:

 1. A macro with argument: LLVM_PACKED(StructDefinition)
 2. A pair of macros defining scope of region with packing:
    LLVM_PACKED_START
     struct A { ... };
     struct B { ... };
    LLVM_PACKED_END

Differential Revision: http://reviews.llvm.org/D14337

llvm-svn: 252099