[analyzer] Fix crash when building CFG with variable of incomplete type

Summary:
I've included a unit test with a function template containing a variable
of incomplete type. Clang compiles this without errors (the standard
does not require a diagnostic in this case). Without the fix, this case
triggers the crash.

Reviewers: klimek

Reviewed By: klimek

Subscribers: cfe-commits

Differential Revision: https://reviews.llvm.org/D30636

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@297129 91177308-0d34-0410-b5e6-96231b3b80d8