Define portable macros for packed struct definitions:

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

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@252099 91177308-0d34-0410-b5e6-96231b3b80d8