[Support] Remove NonowningIslPtr. NFC.

NonowningIslPtr<isl_X> was used as types of function parameters when the
function does not consume the isl object, i.e. an __isl_keep parameter.

The alternatives are:

1. IslPtr<isl_X>
   This has additional calls to isl_X_copy and isl_X_free to
   increase/decrease the reference counter even though not needed. The
   caller already owns a reference to the isl object.

2. const IslPtr<isl_X>&
   This does not change the reference counter, but requires an
   additional load to get the pointer to the isl object (instead of just
   passing the pointer itself).
   Moreover, the compiler cannot rely on the constness of the pointer
   and has to reload the pointer every time it writes to memory (unless
   alias analysis such as TBAA says it is not possible).

The isl C++ bindings currently in development do not have an equivalent
to NonowningIslPtr and adding one would make the binding more
complicated and its advantage in performance is small. In order to
simplify the transition to these C++ bindings, remove NonowningIslPtr.
Change every former use of it to alternative 2 mentioned aboce
(const IslPtr<isl_X>&).

llvm-svn: 295998

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