Debug Info: Represent local anonymous unions as anonymous unions

and as artificial local variables in the debug info.

This is a follow-up to r236059. We can't get rid of the local variables
entirely because the gdb buildbot depends on them, but we can mark them
as artificial while still emitting the correct debug info. As I learned
from review comments other compilers also follow this model.

A paired commit in LLVM temporarily relaxes the debug info verifier to
not check the integrity of DW_OP_bit_pieces of artificial variables.

rdar://problem/20730771

llvm-svn: 236125