[analyzer] Improve test from r207486.

[analyzer] Improve test from r207486.

The constructor that comes right before a variable declaration in the CFG might
not be the initialization of that variable. Previously, we just checked that
the variable's initializer expression was different from the construction
expression, but forgot to see whether the variable had an initializer expression
at all.

Thanks for the prompting, David.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@207562 91177308-0d34-0410-b5e6-96231b3b80d8