[analyzer] Use the expression’s type instead of region’s type in ArrayToPointer decay evaluation

This gives slightly better precision, specifically, in cases where a non-typed region represents the array
or when the type is a non-array type, which can happen when an array is a result of a reinterpret_cast.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@182810 91177308-0d34-0410-b5e6-96231b3b80d8