AST: Fix the linkage of static vars in fn template specializations

We that static variables in function template specializations were
externally visible.  The manglers assumed that externally visible static
variables were numbered in Sema.  We would end up mangling static
variables in the same specialization with the same mangling number which
would give all of them the same name.

This fixes PR21904.

llvm-svn: 224316