[analyzer] do not crash on cases where an array subscript is an rvalue

Array subscript is almost always an lvalue, except for a few cases where
it is not, such as a subscript into an Objective-C property, or a
return from the function.
This commit prevents crashing in such cases.

Fixes rdar://34829842

Differential Revision: https://reviews.llvm.org/D40584

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@319834 91177308-0d34-0410-b5e6-96231b3b80d8