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: 0ff597fd157e6f4fc38580e8d64ab130330d2411@252099 91177308-0d34-0410-b5e6-96231b3b80d8

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