Add default region generator

This hopefully helps to to address the following compile error on our buildbots

BlockGenerators.h:683:7: error: looser throw specifier for ‘virtual
polly::RegionGenerator::~RegionGenerator()’
BlockGenerators.h:164:11: error:   overriding ‘virtual polly::BlockGenerator::~BlockGenerator() noexcept
(true)`

llvm-svn: 248812