Split up the ParseOptionalAttrs method into three different methods for each

Split up the ParseOptionalAttrs method into three different methods for each
class of attributes. This makes it much easier to check for errors and to reuse
the code.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@169336 91177308-0d34-0410-b5e6-96231b3b80d8