[libc++][AIX] Make basic_string layout compatible with earlier version

Summary:
Patch D123580 changed to use bit fields for strings in long and short mode. As a result, this changes the layout of these strings on AIX because bit fields on AIX are 4 bytes, which breaks the ABI compatibility with earlier strings before the change on AIX. This patch uses the attribute 'packed' and anonymous structure to make string layout compatible. This patch will also make test cases alignof.compile.pass.cpp and sizeof.compile.pass.cpp introduced in D127672 pass on AIX.

Reviewed by: philnik, Mordante, hubert.reinterpretcast, libc++

Differential Revision: https://reviews.llvm.org/D128285