Attribute tablegen now understands that attribute arguments can be optional.  This allows for automated checking of the number of arguments expected vs number of arguments given for attributes.  Greatly reduces the amount of manual checking required.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@190368 91177308-0d34-0410-b5e6-96231b3b80d8