stop x3::lit from updating passed container
as x3::lit() is fixes no need to omit[] it anymore
make x3::omit[] fail at compile time if it is called with non-unused_type attr

It should never be called with anything but unused_type attribute
anyway, but if something did it due to error it is better to be safe and
fail at compile time rather than unexpectedly  update attribute, which
could lead to hard to detect errors.
Merge branch 'lit-leaks-attribute' of https://github.com/redbaron/spirit into redbaron-lit-leaks-attribute

Conflicts:
	test/Jamfile
	test/x3/sequence.cpp
Merge branch 'redbaron-lit-leaks-attribute'