[AIX] Pack BasicBlockBits

Summary:
D51664 introduces a new structure BasicBlockBits which it expects to be
packed a certain way. This change is very similar to D60164, and we apply the
same fix:

"On AIX, the canonical layout of bit-fields would cause
these ... to span four bytes. Applying the pack pragma for compilers that
employ the AIX canonical layout allows these ... to fit within the expected
two bytes. In the future, the pragma would also likely need to be applied when
building with Clang on AIX."

Reviewers: jasonliu, hubert.reinterpretcast, sfertile, xingxue

Reviewed By: sfertile

Subscribers: llvm-commits

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D74911