Sema: prevent assertion on stack return checking

In the case that the array indexing itself is within a type dependent context,
bail out of the evaluation.  We would previously try to symbolically evaluate
the expression which would then try to evaluate a non-address expression as an
address, triggering an assertion in Asserts builds.

We only need to consider the array subscript expression itself as in the case
that the base itself being type dependent is handled appropriately in EvalAddr.

Resolves PR26599.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@260867 91177308-0d34-0410-b5e6-96231b3b80d8