Thread Safety Analysis: warn on nonsensical attributes.

Add warnings in cases where an implicit `this` argument is expected to
attributes because either `this` doesn't exist because the attribute is
on a free function, or because `this` is on a type that doesn't have a
corresponding capability/lockable/scoped_lockable attribute.

Reviewers: delesley, aaron.ballman

Differential Revision: https://reviews.llvm.org/D36237

llvm-svn: 310403