Several major fixes, particularly in emitting constant aggregates:

(1) Padding bytes between structure fields (for alignment) were never
    being emitted into the constant pool so the layout did not match!
(2) In printing constants, structures containing structures or arrays
    were never handled.
(3) Support new model for external/uninitialized/initialized globals.
    Uninitialized globals are no longer emitted since they are external.
    Initialized globals may go either in .bss or in .data.

llvm-svn: 4134