[ub,intro.progress] Added entry for UB due to lack of forward progress

[timuraudio]

Added \ubdef{intro.progress.stops} to [intro.progress]. Added matching entry with code example to [ub]. Note: the wording in [ub] already matches the new wording proposed in CWG2816. When the resolution in CWG2816 is applied to [intro.progress], make sure to not lose the \ubdef there.

[leni536]

You might want to add to the explicitly called out phrases at the beginning of [ub.general]:

draft/source/ub.tex

Lines 6 to 9 in f489b81

	This Annex documents undefined behavior explicitly called out in the main standard text using the
	following phrases: the behavior of the program is undefined, has undefined behavior, results in
	undefined behavior, the behavior is undefined, have undefined behavior, is undefined, result has
	undefined behavior. Undefined behavior that is implicit is not covered by this annex. Each entry contains