clang-tidy: Add llvm namespace comments

llvm commonly adds a comment to the closing brace of a namespace to indicate
which namespace is closed. clang-tidy provides with llvm-namespace-comment
a handy tool to check for this habit. We use it to ensure we consitently use
namespace comments in Polly.

There are slightly different styles in how namespaces are closed in LLVM. As
there is no large difference between the different comment styles we go for the
style clang-tidy suggests by default.

To reproduce this fix run:

for i in `ls tools/polly/lib/*/*.cpp`; \
  clang-tidy -checks='-*,llvm-namespace-comment' -p build $i -fix \
  -header-filter=".*"; \
done

This cleanup was suggested by Eugene Zelenko <eugene.zelenko@gmail.com> in
http://reviews.llvm.org/D21488 and was split out to increase readability.

llvm-svn: 273621