Use own class for storing the RejectLogs

Use a container class to store the reject logs. Delegating most calls to
the internal std::map and add a few convenient shortcuts (e.g.,
hasErrors()).

git-svn-id: https://llvm.org/svn/llvm-project/polly/trunk@211780 91177308-0d34-0410-b5e6-96231b3b80d8