Silence GCC 9 error about deprecated implicit copy constructor.

Replacing push_back() with emplace_back() eliminates the calls to the
copy constructor, but I still had to provide explicit copy constructors
because of the call to vector::reserve(), which tries to instantiate them
even though they'll never actually be called when reserve() is called
on an empty vector.

gold/
	* incremental.cc (Sized_incremental_binary::setup_readers): Use
	emplace_back for GCC 5 and later.
	* incremental.h (Incremental_binary::Input_reader): Provide copy
	constructor.
	(Sized_incremental_binary::Sized_input_reader): Likewise.