Add a Subjects line to NoDebugAttr [NFC].

The 'nodebug' attribute had hand-coded constraints; replace those with
a Subjects line in Attr.td.
Also add a missing test to verify the attribute is okay on an
Objective-C method.

Differential Revision: http://reviews.llvm.org/D19689


git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@268065 91177308-0d34-0410-b5e6-96231b3b80d8