Fix the storage class of method instantiations.

We keep the "as written" storage class, but that is a fuzzy concept for
instantiations. With this patch instantiations of methods of class templates
now get a storage class that is based on the semantics of isStatic(). With this
can simplify isStatic() itself.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@179521 91177308-0d34-0410-b5e6-96231b3b80d8