PR37275 packed attribute should not apply to base classes

Clang incorrectly applied the packed attribute to base classes. Per GCC's
documentation and as can be observed from its behavior, packed only applies to
members, not base classes.

This change is conditioned behind -fclang-abi-compat so that an ABI break can
be avoided by users if desired.

Differential Revision: https://reviews.llvm.org/D46218

llvm-svn: 331136