Correctly propagate the storage class to function template instantiations.

This fixes pr15753. This is another case of the fuzzy definition of the
"as written" storage class of an instantiation.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@179581 91177308-0d34-0410-b5e6-96231b3b80d8