[ASTMatchers] Make hasOverloadedOperatorName also match freestanding overloads.

Freestanding overloads are represented as FunctionDecls in the AST, make
the matcher also match them.

Differential Revision: http://reviews.llvm.org/D4493

git-svn-id: http://llvm.org/svn/llvm-project/cfe/trunk@212940 91177308-0d34-0410-b5e6-96231b3b80d8