go/gcimporter15: adopt changes from golang.org/cl/22714

Fixes x/tools build.

Change-Id: Ifd958ff1293ec2c4fd971813ae7b199b5d8244fe
Reviewed-on: https://go-review.googlesource.com/22721
Reviewed-by: Alan Donovan <adonovan@google.com>

go/gcimporter15/bexport.go

@@ -490,8 +490,11 @@ func (p *exporter) paramList(params *types.Tuple, variadic bool) {
 		}
 		p.typ(t)
 		if n > 0 {
-			p.string(q.Name())
+			name := q.Name()
-			p.pkg(q.Pkg(), false)
+			p.string(name)
+			if name != "_" {
+				p.pkg(q.Pkg(), false)
+			}
 		}
 		p.string("") // no compiler-specific info
 	}

go/gcimporter15/bimport.go

@@ -510,10 +510,12 @@ func (p *importer) param(named bool) (*types.Var, bool) {
 		if name == "" {
 			panic("expected named parameter")
 		}
+		if name != "_" {
+			pkg = p.pkg()
+		}
 		if i := strings.Index(name, "·"); i > 0 {
 			name = name[:i] // cut off gc-specific parameter numbering
 		}
-		pkg = p.pkg()
 	}
 
 	// read and discard compiler-specific info