[Support] Complete ISL annotations to IslPtr<>. NFC.

Add missing __isl_(give/take/keep) annotations to IslPtr<> and NonowningIslPtr<>
methods.

Because IslPtr's constructor's annotation would depend on the TakeOwnership
parameter, the parameter has been removed. Caller must copy the object
themselves if the do not want to take ownership.

llvm-svn: 282883