Fixed Assert In CGRecordLowering

Prior to this patch, CGRecordLower assumed that virtual bases could not 
be placed before the nvsize of an object.  This isn't true in Itanium 
mode, virtual bases are placed at dsize rather than vnsize and in the 
case of zero sized non-virtual bases nvsize can be larger than dsize.  
This patch fixes CGRecordLowering to avoid an assert and to clip 
bitfields properly in this case.  A test case is included.

llvm-svn: 207280