This behaves similar to the __has_cpp_attribute builtin macro in that it allows users to detect whether an attribute is supported with the [[]] spelling syntax, which can be enabled in C with -fdouble-square-bracket-attributes.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@320088 91177308-0d34-0410-b5e6-96231b3b80d8