[CodeView] Implement support for bitfields in LLVM

CodeView need to know the offset of the storage allocation for a
bitfield.  Encode this via the "extraData" field in DIDerivedType and
introduced a new flag, DIFlagBitField, to indicate whether or not a
member is a bitfield.

This fixes PR28162.

Differential Revision: http://reviews.llvm.org/D21782

llvm-svn: 274200