Adjust a ValueObjectChild's offset when the child is a bitfield

If a bitfield doesn't fit into the child_byte_size'd window at
child_byte_offset, move the window forward until it fits.  The problem
here is that Value has no notion of bitfields and thus the Value's
DataExtractor is sized like the bitfields CompilerType; a sequence of
bitfields, however, can be larger than their underlying type.

This was not in the big-endian-derived DWARF 2 bitfield attributes
because their offsets were counted from the end of the window, so they
always fit.

rdar://problem/53132189

Differential Revision: https://reviews.llvm.org/D65492

llvm-svn: 368226