cmd/compile: inconsistent error message when identifier begins with invalid character

[smasher164]

What version of Go are you using (go version)?

go version go1.15.2 darwin/amd64

What did you do?

I tried to compile the following program: https://play.golang.org/p/379msfdOuwi

package main

func main() {
	var 3x = 3
	3x := 3  
	var $x = 3
	$x := 3
}

What did you expect to see?

./prog.go:4:6: syntax error: unexpected literal 3, expecting name
./prog.go:5:3: syntax error: unexpected literal 3, expecting name
./prog.go:6:6: invalid character U+0024 '$'
./prog.go:7:2: invalid character U+0024 '$'

What did you see instead?

./prog.go:4:6: syntax error: unexpected literal 3, expecting name
./prog.go:5:3: syntax error: unexpected x at end of statement
./prog.go:6:6: invalid character U+0024 '$'
./prog.go:7:2: invalid character U+0024 '$'
./prog.go:7:5: no new variables on left side of :=

[rsc]

duplicate of #68183.

[griesemer]

Reopening because it's not quite the same issue as #68183.