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>

transform/testdata/allocs2.go

@@ -10,7 +10,7 @@ func main() {
 	derefInt(&n1)
 
 	// This should eventually be modified to not escape.
-	n2 := 6 // OUT: allocs2.go:52.1,52.42 1 0
+	n2 := 6
 	returnIntPtr(&n2)
 
 	s1 := make([]int, 3)
@@ -20,22 +20,22 @@ func main() {
 	readIntSlice(s2[:])
 
 	// This should also be modified to not escape.
-	s3 := make([]int, 3) // OUT: allocs2.go:51.1,51.42 1 0
+	s3 := make([]int, 3)
 	returnIntSlice(s3)
 
-	useSlice(make([]int, getUnknownNumber())) // OUT: allocs2.go:48.1,48.55 1 0
+	useSlice(make([]int, getUnknownNumber()))
 
-	s4 := make([]byte, 300) // OUT: allocs2.go:46.1,46.56 1 0
+	s4 := make([]byte, 300)
 	readByteSlice(s4)
 
-	s5 := make([]int, 4) // OUT: allocs2.go:38.1,38.56 1 0
+	s5 := make([]int, 4)
 	_ = append(s5, 5)
 
 	s6 := make([]int, 3)
 	s7 := []int{1, 2, 3}
 	copySlice(s6, s7)
 
-	c1 := getComplex128() // OUT: allocs2.go:31.1,31.55 1 0
+	c1 := getComplex128()
 	useInterface(c1)
 
 	n3 := 5
@@ -43,13 +43,13 @@ func main() {
 		return n3
 	}()
 
-	callVariadic(3, 5, 8) // OUT: allocs2.go:28.1,28.58 1 0
+	callVariadic(3, 5, 8)
 
-	s8 := []int{3, 5, 8} // OUT: allocs2.go:26.1,26.76 1 0
+	s8 := []int{3, 5, 8}
 	callVariadic(s8...)
 
-	n4 := 3 // OUT: allocs2.go:23.1,23.55 1 0
-	n5 := 7 // OUT: allocs2.go:13.1,13.42 1 0
+	n4 := 3
+	n5 := 7
 	func() {
 		n4 = n5
 	}()