[include-fixer] do not index friend function declaration.

Summary:
we want to exclude friend declaration, but the `DeclContext` of a
friend function declaration is not the class in which it is declared, so we need
to explicitly check if the parent is a `friendDecl`.

Reviewers: bkramer

Subscribers: cfe-commits

Differential Revision: http://reviews.llvm.org/D21175

llvm-svn: 272261