[GCC] PR23529 Mangler part of attrbute abi_tag support

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

Difference between original and this one:

• fixed all failing tests
• fixed mangling for global variable outside namespace
• emit ABI tags for guards and local names
• clang-format + other stylistic changes
• significantly reworked patch according to Richard's suggestions

Sema part, committed before http://reviews.llvm.org/D17567

Differential revision: http://reviews.llvm.org/D18035