[clang-tidy] Added a check for forward declaration in the potentially wrong namespace

Adds a new check "misc-forward-declaration-namespace".
In check, A forward declaration is considerred in a potentially wrong namespace
if there is any definition/declaration with the same name exists in a different
namespace.

Reviewers: akuegel, hokein, alexfh

Patch by Eric Liu!

Differential Revision: http://reviews.llvm.org/D17195

llvm-svn: 261737