NLL: turn on borrowck=migrate by default on 2015 edition

[pnkfelix]

As a sub-step towards having NLL produce hard errors (rather than downgrading its errors to warnings) and removing the AST-borrowck code, we want to turn on the NLL migration mode (introduced with the 2018 edition) so that it operates in all editions. Namely, we want to introduce the -Z borrowck=migrate (and associated -Z two-phase-borrows) on the 2015 edition.

This is a subtask of #43234

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On Zulip, @matthewjasper composed the following interesting table summarizing the open issues related to this, which I am transcribing here (with some light editing to replace pronouns with names).

all blocking issues are marked either P-High or NLL-Sound of those issues the current state is:

Issue number	Blocks migrate	Assigned To	Notes
#46702	Yes	@matthewjasper / @nikomatsakis	Fixed by #57714
#56254	Yes	@matthewjasper	#58739
#57170	Yes	@matthewjasper	Fixed by #57202
#57731	Yes	@matthewjasper	Fixed by #57714
#58127	Yes	@matthewjasper	Fixed by #58347
#58178	Yes No	@matthewjasper	Good enough after #58347
#54105	No	@nikomatsakis	Needs to be fixed, but not clear what, if any, code this affects
#54940	No	@nikomatsakis	Obscure, no easy fix
#46901	No	@pnkfelix	need some idea what we're allowing for 2 phase borrows; (note #56254 above)
#54943	No	@matthewjasper	Fixed as part of #57714
#56993	No	@nikomatsakis	Just a test issue now
#57374	No	@lqd / @nikomatsakis	Not an issue in migrate mode

[pnkfelix]

(Also on Zulip, @matthewjasper stated that they think this should be possible to do for the 1.34 release (beta cut on 2019-02-28, release on 2019-04-11).)

[matthewjasper]

I've updated the table to the current state.

[pnkfelix]

triage: P-high