cmd/compile: set correct line number for multiple defaults in switch error

Fixes #15911.

Change-Id: I500533484de61aa09abe4cecb010445e3176324e
Reviewed-on: https://go-review.googlesource.com/26760
TryBot-Result: Gobot Gobot <gobot@golang.org>
Run-TryBot: Josh Bleecher Snyder <josharian@gmail.com>
Reviewed-by: Matthew Dempsky <mdempsky@google.com>

src/cmd/compile/internal/gc/swt.go

@@ -107,6 +107,7 @@ func typecheckswitch(n *Node) {
 		if ncase.List.Len() == 0 {
 			// default
 			if def != nil {
+				setlineno(ncase)
 				Yyerror("multiple defaults in switch (first at %v)", def.Line())
 			} else {
 				def = ncase

test/switch6.go

@@ -21,12 +21,12 @@ func f0(e error) {
 
 // Verify that the compiler rejects multiple default cases.
 func f1(e interface{}) {
-	switch e { // ERROR "multiple defaults in switch"
-	default:
+	switch e {
 	default:
+	default: // ERROR "multiple defaults in switch"
 	}
-	switch e.(type) { // ERROR "multiple defaults in switch"
-	default:
+	switch e.(type) {
 	default:
+	default: // ERROR "multiple defaults in switch"
 	}
 }