Debug Info: One more bitfield bugfix. While yesterday's r240853 fixed

the DW_AT_bit_offset computation, the byte offset is in fact also
endian-dependent as it needs to point to the storage unit containing the
most-significant bit of the the bitfield.
I'm so looking forward to emitting the endian-agnostic DWARF 3 version
instead.

llvm-svn: 240890