go.tools/go/types: print meaningful panic for unknown expressions

LGTM=iant
R=iant
CC=golang-codereviews
https://golang.org/cl/81310045

go/types/expr.go

@@ -7,6 +7,7 @@
 package types
 
 import (
+	"fmt"
 	"go/ast"
 	"go/token"
 	"math"
@@ -1386,10 +1387,7 @@ func (check *checker) exprInternal(x *operand, e ast.Expr, hint Type) exprKind {
 		// types, which are comparatively rare.
 
 	default:
-		if debug {
-			check.dump("expr = %v (%T)", e, e)
-		}
-		unreachable()
+		panic(fmt.Sprintf("%s: unknown expression type %T", check.fset.Position(e.Pos()), e))
 	}
 
 	// everything went well