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

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.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@172382 91177308-0d34-0410-b5e6-96231b3b80d8