Let constraint manager inform checkers that some assumption logic has happend.

Let constraint manager inform checkers that some assumption logic has happend.
Add new states for symbolic regions tracked by malloc checker. This enables us
to do malloc checking more accurately. See test case.

Based on Lei Zhang's patch and discussion.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@92342 91177308-0d34-0410-b5e6-96231b3b80d8