Fix and stylize the emission of GC/ARC ivar and GC block layout strings.

Specifically, handle under-aligned object references (by explicitly
ignoring them, because this just isn't representable in the format;
yes, this means that GC silently ignores such references), descend
into anonymous structs and unions, stop classifying fields of
pointer-to-strong/weak type as strong/weak in ARC mode, and emit
skips to cover the entirety of block layouts in GC mode.  As a
cleanup, extract this code into a helper class, avoid a number of
unnecessary copies and layout queries, generate skips implicitly
instead of explicitly tracking them, and clarify the bitmap-creation
logic.

llvm-svn: 250919