The MSP430Interrupt attribute does have a sema handler (it's in TargetAttributesSema). Added a FIXME about the attribute being nameless when it really does have a valid name, and a comment explaining why we're using the name instead of the attribute kind.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@196420 91177308-0d34-0410-b5e6-96231b3b80d8