[isl++] Export (almost) all functions from isl

This commit exports the majority of the isl functions to the isl C++ interface.

The official isl C++ bindings still require discussions to define the set of
functions that are officially supported. As a result, the officially exported
functionality will be rather limited until these discussions conclude and a
non-trivial set of isl functions is officially supported through the isl C++
bindings. Starting from this commit we ship with Polly an extended version of
the official isl C++ bindings to ensure sufficient functionality is available
such that LLVM developers can make efficient use of isl through C++. The
practical experience Polly gathers with its bindings will then be used to
gradually upstream patches to isl to extend the official bindings.

llvm-svn: 303506