Till now, preserve_mostcc/preserve_allcc calling convention attributes were only
available at the LLVM IR level. This patch adds attributes for
preserve_mostcc/preserve_allcc calling conventions to the C/C++ front-end.

The code was mostly written by Juergen Ributzka.
I just added support for the AArch64 target and tests.

Differential Revision: http://reviews.llvm.org/D18025

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@263647 91177308-0d34-0410-b5e6-96231b3b80d8