go/internal/gcimporter: replace assert with more helpful panic

If this code is misused, the result of the assert is

	panic: assertion failed

This CL replaces the panic text with a useful message.

Change-Id: I3710ebe975e7f1e907b0378e9c67dd0c224492fb
Reviewed-on: https://go-review.googlesource.com/c/tools/+/310513
Trust: Russ Cox <[email protected]>
Run-TryBot: Russ Cox <[email protected]>
Reviewed-by: Austin Clements <[email protected]>
Auto-Submit: Russ Cox <[email protected]>
gopls-CI: kokoro <[email protected]>
TryBot-Result: Gopher Robot <[email protected]>

Author: rsc

go/internal/gcimporter/iexport.go

@@ -251,7 +251,10 @@ func (p *iexporter) stringOff(s string) uint64 {
 // pushDecl adds n to the declaration work queue, if not already present.
 func (p *iexporter) pushDecl(obj types.Object) {
 	// Package unsafe is known to the compiler and predeclared.
-	assert(obj.Pkg() != types.Unsafe)
+	// Caller should not ask us to do export it.
+	if obj.Pkg() == types.Unsafe {
+		panic("cannot export package unsafe")
+	}
 
 	if _, ok := p.declIndex[obj]; ok {
 		return