Find places where OUString and OString are passed by value.

It's not very efficient, because we generally end up copying it twice -
once into the parameter and again into the destination OUString.

So I create a clang plugin that finds such places and generates a
warning so that we can convert them to pass-by-reference.

Change-Id: I5341a6ea9e3190f4b4c05c42c85595e3dcd83361