IRgen: Enhance CGBitFieldInfo with enough information to fully describe the...

IRgen: Enhance CGBitFieldInfo with enough information to fully describe the "policy" with which a bit-field should be accessed.
 - For now, these policies are computed to match the current IRgen strategy, although the new information isn't being used yet (except in -fdump-record-layouts).

 - Design comments appreciated.

llvm-svn: 101178