Introduced an attribute syntax-neutral method for parsing attribute arguments that is currently being used by GNU and C++-style attributes. This allows C++11 attributes with argument lists to be handled properly, fixing the "deprecated", "type_visibility", and capability-related attributes with arguments.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@205226 91177308-0d34-0410-b5e6-96231b3b80d8