IsExact shouldn't be set to true in WeakObjectProfileTy::getBaseInfo
when the receiver is a class because having a class as the receiver
doesn't guarantee that the Base is exact.

This is a follow-up to r263818.

rdar://problem/25208167


git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@264025 91177308-0d34-0410-b5e6-96231b3b80d8