Diagnose the use of abstract types as array element types. Previously,

Diagnose the use of abstract types as array element types. Previously,
we were relying on checking for abstract class types when an array
type was actually used to declare a variable, parameter, etc. However,
we need to check when the construct the array for, e.g., SFINAE
purposes (see DR337). Fixes problems with Boost's is_abstract type
trait.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@102452 91177308-0d34-0410-b5e6-96231b3b80d8