Factor out custom parsing for iboutletcollection and vec_type_hint attributes

Factor out custom parsing for iboutletcollection and vec_type_hint attributes
into a separate "parse an attribute that takes a type argument" codepath. This
results in both codepaths being a lot cleaner and simpler, and fixes some bugs
where the type argument handling bled into the expression argument handling and
caused us to both accept invalid and reject valid attribute arguments.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@193731 91177308-0d34-0410-b5e6-96231b3b80d8