cmd/compile: add explanations to escape-analysis JSON/LSP logging

For 1.15.

From the test:

{"range":{"start":{"line":7,"character":13},"end":{...},"severity":3,"code":"leaks","source":"go compiler","message":"parameter z leaks to ~r2 with derefs=0","relatedInformation":[
	{"location":{"uri":"file://T/file.go","range":{"start":{"line":9,"character":13},"end":{...}},"message":"escflow:    flow: y = z:"},
	{"location":{"uri":"file://T/file.go","range":{"start":{"line":9,"character":13},"end":{...}},"message":"escflow:      from y = \u003cN\u003e (assign-pair)"},
	{"location":{"uri":"file://T/file.go","range":{"start":{"line":9,"character":13},"end":{...}},"message":"escflow:    flow: ~r1 = y:"},
	{"location":{"uri":"file://T/file.go","range":{"start":{"line":4,"character":11},"end":{...}},"message":"inlineLoc"},

	{"location":{"uri":"file://T/file.go","range":{"start":{"line":9,"character":13},"end":{...}},"message":"escflow:      from y.b (dot of pointer)"},
	{"location":{"uri":"file://T/file.go","range":{"start":{"line":4,"character":11},"end":{...}},"message":"inlineLoc"},

	{"location":{"uri":"file://T/file.go","range":{"start":{"line":9,"character":13},"end":{...}},"message":"escflow:      from \u0026y.b (address-of)"},
	{"location":{"uri":"file://T/file.go","range":{"start":{"line":4,"character":9},"end":...}},"message":"inlineLoc"},

	{"location":{"uri":"file://T/file.go","range":{"start":{"line":9,"character":13},"end":{...}},"message":"escflow:      from ~r1 = \u003cN\u003e (assign-pair)"},
	{"location":{"uri":"file://T/file.go","range":{"start":{"line":9,"character":3},"end":...}},"message":"escflow:    flow: ~r2 = ~r1:"},
	{"location":{"uri":"file://T/file.go","range":{"start":{"line":9,"character":3},"end":...}},"message":"escflow:      from return (*int)(~r1) (return)"}]}

Change-Id: Idf02438801f63e487c35a928cf5a0b6d3cc48674
Reviewed-on: https://go-review.googlesource.com/c/go/+/206658


Run-TryBot: David Chase <drchase@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Cherry Zhang <cherryyz@google.com>