Do not emit duplicate global initializers for template static data members inside namespaces

A quirk of AST representation leads to class template static data member
definitions being visited twice during Clang IRGen resulting in
duplicate (benign) initializers.

Discovered while investigating a possibly-related debug info bug tickled
by the duplicate emission of these members & their associated debug
info.

With thanks to Richard Smith for help investigating, understanding, and
helping with the fix.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@189996 91177308-0d34-0410-b5e6-96231b3b80d8