[analyzer] Relax an assertion in VisitLvalArraySubscriptExpr

The analyzer thinks that ArraySubscriptExpr cannot be an r-value (ever).
However, it can be in some corner cases. Specifically, C forbids expressions
of unqualified void type from being l-values.

Note, the analyzer will keep modeling the subscript expr as an l-value. The
analyzer should be treating void* as a char array
(https://gcc.gnu.org/onlinedocs/gcc-4.3.0/gcc/Pointer-Arith.html).

llvm-svn: 228249