Add ArrayRef<> interface to get the parameters of a FunctionDecl/ObjCMethodDecl.

This is an alternate interface to the separate iterator interfaces
provided by FunctionDecl and ObjCMethodDecl, which precede ArrayRef.

Providing this new interface is more convenient for some uses, and
likely the old interfaces should be removed.  I have not undertaken
that effort in this change because this API introduction may
solicit commentary and that was a larger change than I wanted
to introduce all at once to serve a direct purpose.

llvm-svn: 199466