cmd/compile: don't statically copy string-typed variables

During package initialization, the compiler tries to optimize:

    var A = "foo"
    var B = A

into

    var A = "foo"
    var B = "foo"

so that we can statically initialize both A and B and skip emitting
dynamic initialization code to assign "B = A".

However, this isn't safe in the presence of cmd/link's -X flag, which
might overwrite an initialized string-typed variable at link time. In
particular, if cmd/link changes A's static initialization, it won't
know it also needs to change B's static initialization.

To address this, this CL disables this optimization for string-typed
variables.

Fixes #34675.

Change-Id: I1c18f3b855f6d7114aeb39f96aaaf1b452b88236
Reviewed-on: https://go-review.googlesource.com/c/go/+/198657
Run-TryBot: Matthew Dempsky <mdempsky@google.com>
Reviewed-by: Cuong Manh Le <cuong.manhle.vn@gmail.com>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>

test/linkx.go

@@ -14,10 +14,19 @@ import "fmt"
 var tbd string
 var overwrite string = "dibs"
 
+var tbdcopy = tbd
+var overwritecopy = overwrite
+var arraycopy = [2]string{tbd, overwrite}
+
 var b bool
 var x int
 
 func main() {
 	fmt.Println(tbd)
+	fmt.Println(tbdcopy)
+	fmt.Println(arraycopy[0])
+
 	fmt.Println(overwrite)
+	fmt.Println(overwritecopy)
+	fmt.Println(arraycopy[1])
 }

test/linkx_run.go

@@ -36,7 +36,7 @@ func test(sep string) {
 		os.Exit(1)
 	}
 
-	want := "hello\ntrumped\n"
+	want := "hello\nhello\nhello\ntrumped\ntrumped\ntrumped\n"
 	got := out.String()
 	if got != want {
 		fmt.Printf("got %q want %q\n", got, want)