[analyzer] Refactor and cleanup IsCompleteType

There are three copies of IsCompleteType(...) functions in CSA and all
of them are incomplete (I experienced  crashes in some CSA's test cases).
I have replaced these function calls with Type::isIncompleteType() calls.

A patch by Aleksei Sidorin!

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@219026 91177308-0d34-0410-b5e6-96231b3b80d8