Add method interface to isl C++ bindings

The isl C++ binding method interface introduces a thin C++ layer that allows
to call isl methods directly on the memory managed C++ objects. This makes the
relevant methods directly available via code-completion interfaces, allows for
the use of overloading, conversion constructors, and many other nice C++
features that make using isl a lot easier.

The individual features will be highlighted in the subsequent commits.

Tags: #polly

Reviewed By: Meinersbur

Differential Revision: https://reviews.llvm.org/D30616

llvm-svn: 297462