cmd/cgo: use first error position instead of last one

Just like https://golang.org/cl/34783

Given cgo.go:
     1	package main
     2
     3	/*
     4	long double x = 0;
     5	*/
     6	import "C"
     7
     8	func main() {
     9		_ = C.x
    10		_ = C.x
    11	}

Before:
    ./cgo.go:10:6: unexpected: 16-byte float type - long double

After:
    ./cgo.go:9:6: unexpected: 16-byte float type - long double

The above test case is not portable. So it is tested on only amd64.

Change-Id: If0b84cf73d381a22e2ada71c8e9a6e6ec77ffd2e
Reviewed-on: https://go-review.googlesource.com/54950Reviewed-by: Ian Lance Taylor <iant@golang.org>

misc/cgo/errors/err4.go

+// Copyright 2017 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package main
+
+/*
+long double x = 0;
+*/
+import "C"
+
+func main() {
+	_ = C.x // ERROR HERE
+	_ = C.x
+}

misc/cgo/errors/test.bash

@@ -37,6 +37,9 @@ expect() {
 check err1.go
 check err2.go
 check err3.go
+if [ $(go env GOARCH) == "amd64" ]; then # If we find a portable test case, we can remove this.
+	check err4.go
+fi
 check issue7757.go
 check issue8442.go
 check issue11097a.go

src/cmd/cgo/ast.go

@@ -63,6 +63,7 @@ func (f *File) ParseGo(name string, src []byte) {
 
 	f.Package = ast1.Name.Name
 	f.Name = make(map[string]*Name)
+	f.NamePos = make(map[*Name]token.Pos)
 
 	// In ast1, find the import "C" line and get any extra C preamble.
 	sawC := false
@@ -212,6 +213,7 @@ func (f *File) saveRef(n *ast.Expr, context string) {
 			Go: goname,
 		}
 		f.Name[goname] = name
+		f.NamePos[name] = sel.Pos()
 	}
 	f.Ref = append(f.Ref, &Ref{
 		Name:    name,

src/cmd/cgo/gcc.go

@@ -438,14 +438,7 @@ func (p *Package) guessKinds(f *File) []*Name {
 	for i, n := range names {
 		switch sniff[i] &^ notSignedIntConst {
 		default:
-			var tpos token.Pos
-			for _, ref := range f.Ref {
-				if ref.Name == n {
-					tpos = ref.Pos()
-					break
-				}
-			}
-			error_(tpos, "could not determine kind of name for C.%s", fixGo(n.Go))
+			error_(f.NamePos[n], "could not determine kind of name for C.%s", fixGo(n.Go))
 		case notStrLiteral | notType:
 			if sniff[i]&notSignedIntConst != 0 {
 				n.Kind = "uconst"
@@ -543,10 +536,6 @@ func (p *Package) loadDWARF(f *File, names []*Name) {
 
 	// Scan DWARF info for top-level TagVariable entries with AttrName __cgo__i.
 	types := make([]dwarf.Type, len(names))
-	nameToRef := make(map[*Name]*Ref)
-	for _, ref := range f.Ref {
-		nameToRef[ref.Name] = ref
-	}
 	r := d.Reader()
 	for {
 		e, err := r.Next()
@@ -597,10 +586,7 @@ func (p *Package) loadDWARF(f *File, names []*Name) {
 		if types[i] == nil {
 			continue
 		}
-		pos := token.NoPos
-		if ref, ok := nameToRef[n]; ok {
-			pos = ref.Pos()
-		}
+		pos := f.NamePos[n]
 		f, fok := types[i].(*dwarf.FuncType)
 		if n.Kind != "type" && fok {
 			n.Kind = "func"

src/cmd/cgo/main.go

@@ -54,6 +54,7 @@ type File struct {
 	Calls    []*Call             // all calls to C.xxx in AST
 	ExpFunc  []*ExpFunc          // exported functions for this file
 	Name     map[string]*Name    // map from Go name to Name
+	NamePos  map[*Name]token.Pos // map from Name to position of the first reference
 }
 
 func nameKeys(m map[string]*Name) []string {