Improved llvm-namespace-comment check.

Summary:
Handle various forms of existing namespace closing comments, fix
existing comments with wrong namespace name, ignore short namespaces.

The state of this check now seems to be enough to enable it by default to gather
user feedback ;)

Reviewers: klimek

Reviewed By: klimek

Subscribers: cfe-commits

Differential Revision: http://reviews.llvm.org/D3825