[analyzer] IvarInvalidation: add annotation for partial invalidation

The new annotation allows having methods that only partially invalidate
IVars and might not be called from the invalidation methods directly
(instead, are guaranteed to be called before the invalidation occurs).
The checker is going to trust the programmer to call the partial
invalidation method before the invalidator.This is common in cases when
partial object tear down happens before the death of the object.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@174779 91177308-0d34-0410-b5e6-96231b3b80d8