ProfileData: Avoid unnecessary copies of CounterData

We're currently copying CounterData from InstrProfWriter into the
OnDiskHashTable, even though we don't need to, and then carelessly
leaking those copies. A const pointer is much better here.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@207009 91177308-0d34-0410-b5e6-96231b3b80d8