transform: add -print-allocs-cover and restore -print-allocs reason output

PR #5220 changed -print-allocs output to the go coverage tool format, which
replaced the original human-readable explanation of why each object had to be
heap allocated. That explanation is useful on its own, so this restores it as
the default behavior of -print-allocs and moves the coverage format behind a
-print-allocs-cover flag.

Signed-off-by: Piotr Bocheński <piotr@bochen.ski>

compileopts/options.go

@@ -47,6 +47,7 @@ type Options struct {
 	Nobounds                bool
 	PrintSizes              string
 	PrintAllocs             *regexp.Regexp // regexp string
+	PrintAllocsCover        bool           // emit allocs in go coverage tool format
 	PrintStacks             bool
 	Tags                    []string
 	GlobalValues            map[string]map[string]string // map[pkgpath]map[varname]value