Objective-C properties: fix bogus use of "isa<>" on a QualType.

The code used "isa" to check the type and then "getAs" to look through
sugar; we need to look through the sugar when checking, too, otherwise
any kind of sugar (nullability qualifiers in the example; or a
typedef) will thwart this semantic check. Fixes rdar://problem/23804250.

llvm-svn: 255066