clang-format: Add more options to namespace indentation.

With this patch, clang-format can be configured to:
* not indent in namespace at all (former behavior).
* indent in namespace as in other blocks.
* indent only in inner namespaces (as required by WebKit style).

Also fix alignment of access specifiers in WebKit style.

Patch started by Marek Kurdej. Thank you!

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@187540 91177308-0d34-0410-b5e6-96231b3b80d8