Do not use namespace polly inside a header.

In general this fixes ambiguity that can arise from using
a different namespace that declares the same symbols as
we do.

One example inside llvm would be:
  createIndVarSimplifyPass(..);

Which can be found in:
  llvm/Transforms/Scalar.h
and
  polly/LinkAllPasses.h

llvm-svn: 210755