Accept [[gnu::*]] for all __attribute__((*))s which are:

 1) Supported by Clang, and
 2) Supported by GCC, and
 3) Documented in GCC's manual.

g++ allows its C++11-style attributes to appertain only to the entity being
declared, and never to a type (even for a type attribute), so we do the same.

llvm-svn: 172382