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

git-svn-id: https://llvm.org/svn/llvm-project/polly/trunk@210755 91177308-0d34-0410-b5e6-96231b3b80d8