[dwarfdump] Prettyprint DW_AT_APPLE_property_attribute bitfield values.

This change depends on the ApplePropertyString helper that I sent spearately.
Not sure how you want this tested: as a tool test by adding a binary to dump, or as an llvm test starting from an IR file?

Reviewers: dblaikie, samsonov

Subscribers: llvm-commits

Differential Revision: http://reviews.llvm.org/D5689

llvm-svn: 219507