[GCC] PR23529 Sema part of attrbute abi_tag support

Original patch by Stefan Bühler http://reviews.llvm.org/D12834

Difference between original and this one:
- fixed all comments in original code review
- added more tests, all new diagnostics now covered by tests
- moved abi_tag on re-declaration checks to Sema::mergeDeclAttributes
  where they actually may work as designed
- clang-format + other stylistic changes

Mangle part will be sent for review as a separate patch.

Differential Revision: http://reviews.llvm.org/D17567

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@263015 91177308-0d34-0410-b5e6-96231b3b80d8