clang-format: better handle namespace macros

Summary:
Other macros are used to declare namespaces, and should thus be handled
similarly. This is the case for crpcut's TESTSUITE macro, or for
unittest-cpp's SUITE macro:

      TESTSUITE(Foo) {
      TEST(MyFirstTest) {
        assert(0);
      }
      } // TESTSUITE(Foo)

This patch deals with this cases by introducing a new option to specify
lists of namespace macros. Internally, it re-uses the system already in
place for foreach and statement macros, to ensure there is no impact on
performance.

Reviewers: krasimir, djasper, klimek

Reviewed By: klimek

Subscribers: acoomans, cfe-commits, klimek

Tags: #clang

Differential Revision: https://reviews.llvm.org/D37813

llvm-svn: 362740