ARM/APCS: Don't respect bit-field types when laying out structures.

 - This fixes the last known ABI issues with ARM/APCS.

 - I've run the first 1k ABITests with '--no-unsigned --no-vector --no-complex'
   on {armv6, armv7} x {-mno-thumb, -mthumb}, and the first 10k tests for armv7
   -mthumb, for both function return types and single argument calls. These all
   pass now (they failed horribly before without --no-bitfield).

llvm-svn: 102070