Parsing of C++11 attributes:

Parsing of C++11 attributes:
 * Alternative tokens (such as 'compl') are treated as identifiers in
   attribute names.
 * An attribute-list can start with a comma.
 * An ellipsis may not be used with either of our currently-supported
   C++11 attributes.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@154381 91177308-0d34-0410-b5e6-96231b3b80d8