PR25575: Make GCC 4.4+ comatible layout for packed bit-fileds of char type, patch...

PR25575: Make GCC 4.4+ comatible layout for packed bit-fileds of char type, patch by D. Polukhin

This CL is for discussion how to better fix bit-filed layout compatibility issue with GCC (see PR25575 for test case and more details). Current clang behavior is compatible with GCC 4.1-4.3 series but it was fixed in 4.4+. Ignoring packed attribute looks very odd and because it was also fixed in GCC 4.4+, it makes sense also fix it in clang.
Differential Revision: http://reviews.llvm.org/D14872

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@254596 91177308-0d34-0410-b5e6-96231b3b80d8