[unittest] Translate isl tests to C++ bindings

For this translation we introduce two functions, valFromAPInt and APIntFromVal,
to convert between isl::val and APInt. For now these are just proxies, but in
the future they will replace the current isl_val* based conversion functions.

The isl unit test cases benefit most from the new isl::boolean (from Michael
Kruse), which can be explicitly casted to bool and which -- as part of this cast
-- emits a check that no error condition has been triggered so far. This allows
us to simplify

  EXPECT_EQ(isl_bool_true, isl_val_is_zero(IslZero));

to

  EXPECT_TRUE(IslZero.is_zero());

This simplification also becomes very clear in operator==, which changes from

  auto IsEqual = isl_set_is_equal(LHS.keep(), RHS.keep());
  EXPECT_NE(isl_bool_error, IsEqual);
  return IsEqual;

to just

  return bool(LHS.is_equal(RHS));

Some background for non-isl users. The isl C interface has an isl_bool type,
which can be either true, false, or error. Hence, whenever a function returns
a value of type isl_bool, an explicit error check should be considered. By
using isl::boolean, we can just cast the isl::boolean to 'bool' or simply use
the isl::boolean in a context where it will automatically be casted to bool
(e.g., in an if-condition). When doing so, the C++ bindings automatically add
code that verifies that the return value is not an error code. If it is, the
program will warn about this and abort. For cases where errors are expected,
isl::boolean provides checks such as boolean::is_true_or_error() or
boolean::is_true_no_error() to explicitly control program behavior in case of
error conditions.

Thanks to the new automatic memory management, we also can avoid many calls to
isl_*_free. For code that had previously been managed by IslPtr<>, many calls
to give/take/copy are eliminated.

Tags: #polly

Reviewed By: Meinersbur

Differential Revision: https://reviews.llvm.org/D30618

llvm-svn: 297464